Invoicing vs subscription vs usage-based billing (2026 guide)

Key takeaways

• The main difference between the three models is how you calculate what customers owe and when they pay.

• Many growing Singapore businesses use more than one model. Choose the right one for each revenue stream instead of trying to fit everything into a single approach.

• Airwallex brings Invoicing, Subscription Management and Usage-Based Billing together, so you can manage different billing models without juggling separate tools.

Choosing between invoicing, subscription billing and usage-based billing affects how you collect payments, how much time you spend on admin, and how easily your business can grow.

This guide walks you through three questions to help you choose the right model.

You’ll also learn when it might be time to switch, what each model means for GST and InvoiceNow if you’re GST-registered, and how to combine models when one isn’t enough.

Invoicing vs subscription vs usage-based billing: Which model fits?

The main difference between these models is how you calculate what customers owe and when they pay. Here’s a quick overview:

Invoicing lets you bill for work or goods after you deliver them, with an amount that can vary from one sale to the next. Subscription billing charges a set amount on a regular schedule, while usage-based billing calculates the bill from what each customer actually uses, such as API calls or gigabytes stored.

You don’t have to choose just one model for your whole business. Many businesses use different models for different revenue streams. The sections below explain when each one makes sense and how you can combine them.

Option 1: Invoicing

Invoicing works well when the amount you charge changes from one job to the next, or when a customer makes a one-off purchase. But if you’re billing the same customers regularly, it can create extra admin.

When the scope changes from job to job

A marketing agency might price each campaign based on its deliverables, a consultancy might bill by the hour, and a freight forwarder might charge per shipment.

Because the amount varies each time, a separate invoice lets you bill for the work actually done. The customer reviews the charges and pays the invoice.

Recurring invoices are not the same as subscriptions

You can send an invoice every month for a retainer or ongoing service. But unless payment is automated, your customer still needs to pay each invoice.

With subscription billing, the customer agrees to the terms upfront and payments are collected automatically on a set schedule.

Here’s the difference:

If you send the same customers invoices for the same amount every month and spend time following up on payments, subscription billing could reduce the admin involved.

Tax invoice requirements for GST-registered businesses

If your business is GST-registered, your tax invoices generally need to include the details required by the Inland Revenue Authority of Singapore (IRAS).

Set up your invoice template with the required fields from the start to avoid correcting invoices later.

Option 2: Subscription billing

Subscription billing works best when you charge the same amount on a regular schedule. If you know how much each customer owes and when payment is due, you can automate collection and make revenue easier to forecast.

Fixed charges that repeat

A design studio charging a monthly retainer, an HR software company billing per seat, and a membership platform all have something in common: the price is agreed upfront and stays the same each cycle.

Customers authorise payment once, and the charge is collected on schedule. This reduces manual invoicing and makes recurring revenue easier to predict.

Subscription billing is less suitable when charges vary significantly from month to month. You may need to issue additional invoices for work or usage that falls outside the agreed fee.

How recurring collection works in Singapore

PayNow requires customers to initiate each payment, so it suits one-off payments and invoices rather than automatic recurring charges. Cards on file are a common option for scheduled collection, as customers authorise future payments upfront.

GIRO offers another option for automatic collection directly from customers’ bank accounts. However, setup time can vary by method: the Association of Banks in Singapore (ABS) reports that paper applications took an average of three weeks before deductions began, while eGIRO digitises the authorisation process.¹

Check whether your bank supports GIRO collection for your business before choosing this route.

Handling failed payments

Card payments can fail because a card has expired or the customer has insufficient funds. Airwallex can retry failed payments on a schedule and send reminders, reducing the need to follow up on every missed payment manually.

Option 3: Usage-based billing

Usage-based billing works well when customers’ costs should reflect how much they use your product. Instead of paying a fixed fee, they’re charged based on measured consumption.

Charges that follow consumption

Common billing units include application programming interface (API) calls, gigabytes stored and messages sent. AI products may also charge by tokens processed or agent tasks completed.

Since serving heavy users can cost more, a flat fee may leave you undercharging for high usage. Usage-based pricing lets light users pay less while heavy users pay more, giving customers room to start small and scale up without renegotiating their plans.

What you need to run it

Usage-based billing relies on two building blocks: usage events and meters.

Your product records an event each time a customer uses it, while a meter totals those events over the billing period to calculate the charge.

With Airwallex, you can configure meters in the WebApp without code, send usage events through an API and generate invoices automatically. Pricing can be set per unit, by tiers or by volume.

The trade-off to plan for

The main challenge with usage-based billing is that bills and revenue can fluctuate with usage. Customers may find their charges harder to predict, while missed or inaccurate usage events can lead to incorrect invoices.

Real-time usage data and estimated charges help customers track their consumption and understand what they may owe before the billing period ends.

Option 4: Hybrid billing

You don’t have to use one billing model across your entire business. Combining models lets you charge differently for recurring services, one-off work and variable usage.

A base fee plus usage

A software company might charge a fixed monthly fee that includes a set amount of usage, then bill extra for anything above that limit. The subscription covers the base fee, while usage-based charges account for overages.

Customers know what their plan costs, and you can charge more when they use more.

Project work alongside a recurring plan

A corporate secretarial firm might charge a monthly fee for ongoing compliance services and invoice one-off company incorporations separately. This keeps recurring charges predictable without forcing project work into a fixed plan.

Start with the model that suits your main revenue stream, then add another when you need to bill for something different.

Want to use a hybrid pricing structure? Airwallex lets you combine subscription fees with usage-based charges, so you can bill for extra consumption without managing a separate billing process.

How to choose a billing model for your Singapore business

Start with how you charge customers, then choose the billing model that fits. These three questions can help you narrow it down:

1. Is it a one-off sale or project? Invoicing usually fits best when the scope or price varies from job to job.

2. Does the charge depend on measured usage? Use usage-based billing when the charge is calculated from consumption. If you also charge a fixed base fee, consider a hybrid model.

3. Is the recurring amount fixed each cycle? If so, subscription billing may fit. If the amount varies with each job, invoicing may be more suitable.

For usage-based billing, you should be able to define a measurable unit, such as tokens processed, API calls or gigabytes stored. If you can’t, it may be difficult to calculate charges consistently.

What this looks like for Singapore businesses

These examples are illustrative. Choose the business closest to yours, keeping in mind that some businesses use more than one model:

Choosing a pricing model for SaaS

SaaS businesses typically choose from three pricing approaches, each with different trade-offs:

• Flat-rate pricing: Easy for customers to understand, but light and heavy users pay the same amount.

• Per-seat pricing: Charges grow with team size, though the number of seats may not reflect actual usage or value, especially for AI features.

• Usage-based pricing: Charges reflect consumption, but monthly bills can vary.

Some SaaS businesses combine these approaches, such as charging a per-seat subscription with additional fees for AI usage or volume above a set limit.

Signs it’s time to switch billing models

Your billing needs can change as your business grows. Here are some signs that your current model may no longer fit:

Moving from invoicing to subscription

If you send the same customers invoices for the same amount every month, spend time chasing late payments or regularly match payments to invoices by hand, subscription billing could reduce the admin.

Customers can authorise recurring payments upfront, so you spend less time collecting each payment manually.

Moving from subscription to usage-based billing

If your costs rise with customer usage, a flat subscription fee may mean heavy users cost more to serve without paying more. Lighter users may also feel they’re paying for more than they need.

If customers want more flexible pricing, consider adding usage-based charges to your existing subscription rather than replacing it entirely.

4 mistakes to avoid when switching billing models

Here are some common mistakes to avoid when switching models:

1. Putting variable work on a fixed subscription: You may end up adjusting charges or issuing extra invoices manually.

2. Switching to usage-based billing before you can track consumption reliably: Missing or inaccurate usage data can lead to incorrect invoices.

3. Using separate tools for each billing model: This can split customer and revenue data across systems.

4. Moving every customer at once: Test the new model with new customers or a smaller segment before rolling it out more widely.

Changing billing models can also affect when you issue invoices and account for GST. The next section explains what Singapore businesses need to consider.

How your billing model affects GST and InvoiceNow

If your business is GST-registered, you need to account for GST at the right time and meet Singapore’s e-invoicing requirements. Your billing model affects how you issue invoices and record payments, but the general GST rules apply across models.

When you account for GST

For most supplies, GST is accounted for at the earlier of when you issue an invoice or receive payment. A tax invoice to a GST-registered customer must generally be issued within 30 days of the time of supply.²

There is a special rule for certain continuous supplies invoiced up to 12 months in advance, provided the invoice includes the required payment dates and GST details. If you use advance billing for recurring services, check whether this rule applies.²

For more details, see our guide to GST invoice requirements in Singapore.

InvoiceNow requirements

InvoiceNow is Singapore’s nationwide e-invoicing network. The GST InvoiceNow Requirement is being rolled out in phases: it already applies to certain new voluntary GST registrants, with other GST-registered businesses scheduled to follow between 2028 and 2031.³

Check your implementation date and make sure your billing or accounting software supports the required invoice data submissions to IRAS.

For the full timeline and requirements, read our guide to InvoiceNow in Singapore.

Billing overseas customers under each model

If you bill customers overseas, consider how each billing model handles currency, payments and GST.

Currency and payment methods

With invoicing, you can set the currency for each invoice. Subscription billing typically uses an agreed currency for recurring charges, while usage-based bills vary with consumption. Exchange rates and conversion fees can affect the final amount your customer pays.

With Airwallex Billing, you can bill in multiple currencies, accept payments through supported payment methods and hold funds in supported currencies, helping you manage overseas revenue and expenses.

GST on foreign-currency invoices

If your business is GST-registered, invoices issued in a foreign currency must also show the total before GST, the GST amount and the total including GST in Singapore dollars. See our guide to GST invoice requirements in Singapore for the full list of requirements.

Why Singapore businesses use Airwallex for their billing

As your business grows, you may need to invoice project work, collect recurring subscription fees and charge for usage. Managing these through separate tools can split your billing processes and make it harder to track revenue.

That’s where Airwallex comes in. Airwallex Billing brings Invoicing, Subscription Management and Usage-Based Billing together on one platform.

You can automate recurring payments, combine fixed fees with usage-based charges, and invoice one-off work as needed. Start with the model that suits your business today, then add others as your pricing evolves, without switching billing platforms.

You can sign up for an Airwallex account fully online; there are no setup fees or monthly fees on the free Explore plan.

Frequently asked questions

What is the difference between invoicing, subscription billing and usage-based billing?

Invoicing asks the customer to pay an amount you set after the work is done. Subscription billing collects a fixed amount on a schedule after the customer agrees once. Usage-based billing charges for what the customer actually used in the period. The difference comes down to how the amount is set and who triggers payment.

Should my business use invoicing or subscription billing?

Choose invoicing if the amount changes from job to job or the work is one-off. Choose a subscription if the same amount repeats on a schedule. If you send the same customers the same invoice every month and spend time chasing payment, a subscription usually saves effort. Many businesses use both, one for each type of work.

Which billing model is best for a SaaS business in Singapore?

It depends on how your pricing works. Flat-rate or per-seat plans suit subscription billing. If your costs and customer value grow with usage, such as AI features or API calls, usage-based billing fits better. Many SaaS companies combine a base subscription with usage charges. If you’re GST-registered, check how invoice timing affects when you account for GST.

Is a recurring invoice the same as a subscription?

No. A recurring invoice repeats on a schedule, but your customer still has to pay each one. A subscription is authorised once, and payment is collected automatically on each billing date. Recurring invoices suit amounts that shift slightly, and subscriptions suit a fixed amount.

Can I use more than one billing model at the same time?

Yes, and many businesses do. You can charge a base subscription with usage on top, or run a monthly plan alongside one-off invoices for project work. Match each revenue stream to the model that fits it, and keep the models in one tool so your customer and revenue data stays together. Airwallex Billing supports all three.

When should I move from a subscription to usage-based billing?

Move when your costs rise with usage but your price doesn’t, when heavy and light users pay the same, or when customers ask for pay-as-you-go pricing. You don’t have to drop your subscription. You can keep a base fee and add usage on top. Make sure you can measure usage reliably first, because a missed event means a wrong invoice.

Sources:

1. abs.org.sg/e-payments/eGIRO

2. iras.gov.sg/docs/default-source/e-tax/etaxguide_gst_time-of-supply-rules.pdf

3. iras.gov.sg/news-events/newsroom/committee-of-supply-2026--extension-of-gst-invoicenow-requirement-to-all-gst-registered-businesses-by-april-2031