How to send an invoice: Methods, templates & tips (2026 guide)

Key takeaways:

• Knowing how to send an invoice properly affects how fast you get paid and how professional your business looks to customers.

• Whether you should use postal mail, email, or invoicing software depends on your invoice volume and what your client expects.

• Airwallex Invoicing lets you create, send, and track invoices from one platform, while giving customers a simple way to pay online.

Knowing how to send an invoice correctly can be the difference between getting paid on time and chasing overdue payments for weeks.

This guide walks you through the full process: when to send your invoice, which delivery method suits your business, what to include, and how to word your email. You'll also find ready-to-use templates for different invoicing scenarios and tips for following up on late payments.

Whether you're a freelancer, a small business owner, or part of a larger team, getting these details right protects your cash flow and keeps your customer relationships professional.

When to send an invoice

Timing affects both your cash flow and how professional you look to clients. Send an invoice too early, and it can feel presumptuous. Send it too late, and payment gets delayed.

For one-time projects

Send your invoice as soon as you finish the work or deliver the goods. This keeps the job fresh in your customer's mind and lowers the risk of payment delays.

Before sending it, make sure the project is fully complete and any final changes have been agreed on. This helps avoid having to revise the invoice later.

For ongoing services

Invoice at agreed intervals or milestones for ongoing work. This could mean monthly billing, or invoicing after each project phase.

Agree on these intervals with your customer upfront. Clear communication avoids misunderstandings and keeps your invoicing schedule predictable. Regular invoicing also helps you manage cash flow and keeps your customer informed about progress.

What to include on an invoice

Before worrying about how to send an invoice, make sure you've included all the information your customer needs to understand the charge and make the payment. This includes:

Business and customer details

Include your business name, address and contact details, along with the customer's name and address. This makes it clear who issued the invoice and who is being billed.

Invoice identification

Give each invoice a unique invoice number so you and your customer can easily refer to it when tracking or following up on payment. Include the invoice date and date of supply, and add a purchase order number if the customer provided one.

Cost and payment details

List each product or service separately rather than giving one lump-sum figure. Then clearly show the total amount due, payment terms and payment methods.

If you accept online payments, adding a payment link directly to the invoice can make it easier for customers to pay without having to copy bank details or navigate to another page.

Invoicing rules in Singapore: GST and InvoiceNow

If you're running a business in Singapore, there are two invoicing requirements to be aware of: GST tax invoices and InvoiceNow.

GST tax invoice requirements

If you're GST-registered, you generally need to issue a tax invoice when making standard-rated supplies to another GST-registered business. Your invoice should include:

• The words "Tax Invoice"

• Your business name, address and GST registration number

• Invoice number and date

• Customer's name and address

• Description of the goods or services

• GST rate and amount

• Amount payable before and after GST

For invoices of S$1,000 or less including GST, you can use a simplified tax invoice with fewer details. GST invoices and supporting records must generally be kept for at least five years.

InvoiceNow e-invoicing

InvoiceNow is Singapore's nationwide e-invoicing network, built on the Peppol standard. Instead of sending a PDF invoice by email, businesses can send structured invoices directly between compatible accounting systems.

The GST InvoiceNow Requirement is being introduced in phases, starting with newly incorporated companies that voluntarily register for GST and expanding to other GST-registered businesses through 2031.

If you're affected by the requirement, you'll need to use an InvoiceNow-ready accounting or invoicing solution to submit invoice data to IRAS.

3 ways of sending an invoice

There are several ways to get an invoice to your customer, but the right option depends on how quickly you need to get paid, how many invoices you send, and how much of the process you want to automate.

Here’s a quick comparison of the three most common options:

1. Postal mail

Sending a paper invoice by post is the most traditional option and may still be preferred by some clients or industries.

2. Email

Email is the simplest and fastest option for most businesses, letting you send an invoice directly to your customer within seconds.

3. Invoicing software

Invoicing software takes things a step further by letting you create, send, track and follow up on invoices from one place.

How to send an invoice by email

• Prepare the invoice. Create it using a word processor, spreadsheet or invoicing software, and check that all the details are correct.

• Save it as a PDF. This preserves the formatting and makes the invoice harder to edit. Use a clear filename, such as Invoice_12345_ClientName.pdf.

• Write the email. Keep the message short and include the invoice number, amount due and payment deadline.

• Attach the invoice. Make sure you've attached the correct PDF before sending.

• Proofread and send. Check the recipient, attachment and email for any errors.

How to send an invoice using software

• Create the invoice. Start with a template and add the customer's details, line items and payment information.

• Review the invoice. Check the invoice number, amount, due date and payment terms.

• Choose how to send it. Most invoicing platforms can email the invoice directly to the customer, usually with a PDF or payment link.

• Set up reminders. If available, enable automatic reminders for upcoming or overdue payments.

• Track payment. Use the software dashboard to see whether the invoice has been viewed and paid.

Invoice email templates

The right template saves you time and helps you sound professional. Below are templates for four common scenarios:

First-time invoice

Use this template for a new client or a one-off project.

Subject: Invoice for [Project Name] – Invoice #[12345] – Due [Due Date]

Body: [Business logo]

Dear [Customer Name],

Please find attached the invoice for the [project/service] we recently completed for you.

Invoice details:

• Invoice number: [12345]

• Due date: [Due Date]

• Services provided: [list services and cost breakdown]

• Total amount due: [S$1,500.00]

To make payment, you can use this secure [payment link] or transfer the amount to the following bank account:

• Bank name: [Bank Name]

• Account name: [Company Name]

• Account number: [Account Number]

If you have any questions, please contact me at [your email] or [your phone number].

Thanks, Name [Company Address]

Recurring invoice

Use this template for clients you bill on a regular schedule, such as monthly retainers.

Subject: [Month] Invoice for [Service] – Invoice #[12346] – Due [Due Date]

Body: Dear [Customer Name],

As agreed, here's your invoice for [Month]'s [service/retainer].

Invoice details:

• Invoice number: [12346]

• Billing period: [Start Date] – [End Date]

• Due date: [Due Date]

• Total amount due: [S$800.00]

You can pay using this secure [payment link] or via bank transfer using the details on the attached invoice.

Let me know if you have any questions.

Thanks, [Name]

Milestone or project-phase invoice

Use this template when you're billing for a specific stage of a larger project.

Subject: Invoice for [Project Name] – Phase [X] Complete – Due [Due Date]

Body: Dear [Customer Name],

[Phase X] of [Project Name] is now complete. Please find the invoice for this milestone attached.

Invoice details:

• Invoice number: [12347]

• Milestone: [Phase X description]

• Due date: [Due Date]

• Total amount due: [S$3,200.00]

You can pay via this secure [payment link] or bank transfer. Full details are on the attached invoice.

Looking forward to starting on [Phase X+1].

Thanks, [Name]

Friendly reminder before the due date

Use this template a few days before an invoice is due, especially for larger amounts or new clients.

Subject: Reminder: Invoice #[12345] due [Due Date]

Body: Dear [Customer Name],

Just a quick reminder that invoice #[12345] for [S$1,500.00] is due on [Due Date]. I've attached a copy for your reference.

You can pay via this secure [payment link] or the bank details on the invoice.

Let me know if you need anything else to process this.

Thanks, [Name]

When to follow up on an invoice

Following up on an invoice doesn't have to feel awkward. Managing your cash flow well means staying on top of overdue payments, and it's common for payments to run late.

Timing your first reminder

If a payment is overdue, send a follow-up within a week. Keep the tone professional and polite; a simple reminder email can go a long way toward prompting payment.

Always reference the invoice number and due date in your message. This avoids confusion and makes it easy for your customer to locate the invoice and process payment.

For example: "I wanted to follow up on invoice #12345, which was due on [due date]. We haven't yet received payment, and I wanted to check if there were any issues or if you needed additional information."

Automating your reminders

Set up automated reminders if your invoicing software supports it. Many platforms can send a reminder a few days before the due date, then again if the invoice becomes overdue.

This keeps your invoicing process consistent without requiring you to manually track and chase every payment.

Offering flexible payment methods

Offering more than one payment method improves your chances of getting paid on time. Some customers prefer credit card payments, while others prefer local payment methods or bank transfers.

The easier you make it for a customer to pay, the fewer excuses they have to delay.

Using tools to improve your invoicing

The right tools save you time and reduce manual errors in your invoicing process. Two areas make the biggest difference: automating invoice creation and making it easy for customers to pay.

Automate invoice creation

Invoicing software lets you set up templates that automatically populate with customer details, itemised services, and payment terms. This cuts down on manual entry and reduces the risk of mistakes on invoices you send out regularly.

Airwallex Invoicing lets you create and send invoices using different business entities in more than 130 currencies, and schedule them to go out automatically once generated. 

Track invoice status in one place

Most invoicing software gives you a dashboard showing which invoices are settled, pending, or overdue. This visibility helps you spot late payments early and follow up before they become a bigger cash flow problem.

Get paid faster with Payment Links

Airwallex Payment Links let you generate a secure payment link in seconds and share it by email, SMS, or social media, or embed it directly into your invoices. Your customer clicks the link and pays online using their preferred method, so you get your money faster.

Payment Links work with 160+ payment methods, including major cards, digital wallets, and local options, giving your customers flexibility in how they pay.

Make your invoicing effortless with Airwallex

Managing invoices manually can quickly become time-consuming, especially when you're sending invoices regularly or getting paid in different currencies.

With Airwallex, you can create and send invoices, accept payments, and keep track of what you've been paid, all from one platform.

• Create professional invoices: Build and send invoices using customisable templates, with your business and payment details included.

• Get paid online: Let customers pay directly from the invoice using cards, bank transfers and 160+ local payment methods.

• Track payments: See which invoices have been sent, viewed and paid, so you know when to follow up.

• Reconcile payments: Connect Airwallex with accounting platforms such as Xero and QuickBooks to make reconciliation easier.

Frequently asked questions (FAQs)

How do I send an invoice to someone?

The fastest way to send an invoice is by email, as a PDF attachment, with a clear subject line and the invoice number. You can also use invoicing software, which sends the invoice and tracks whether your client has opened or paid it. If your client doesn't use email, a printed invoice sent by mail still works, but it takes longer to arrive.

Do invoices need to be signed?

No, an invoice usually doesn't need a signature to be valid. The contract or purchase order between you and your client is typically the binding legal document, and the invoice is simply a request for payment based on that agreement. Some clients may still ask for a signed copy for their own records, so it's worth checking their preference.

How do I send an invoice as a freelancer or sole trader?

Sending an invoice as a freelancer works the same way as sending one from a registered company. Use your own name and contact details instead of a business address, and include the same core details: invoice number, services provided, and payment terms. The delivery method doesn't change based on your business structure.

Can I send an invoice through WhatsApp or text message?

You can share an invoice link or PDF through WhatsApp or text message, but it shouldn't be your only method. Messaging apps aren't built for storing financial records, and your client may struggle to find the invoice later when they need it for their own accounts. Use messaging as a quick nudge, and rely on email or invoicing software for the actual document.

What should I do if a client doesn't pay an invoice on time?

Start with a polite follow-up email that references the invoice number and due date. If there's still no response after a reasonable period, consider a phone call or a firmer written reminder, and check whether your contract allows for late payment fees. For invoices that stay unpaid for a long time, you may need to escalate to a formal demand letter or debt collection.

How do I send an invoice to an overseas client?

Sending an invoice overseas works the same way as sending one locally; you just need to account for currency and payment method differences. State the currency clearly on the invoice, and offer a payment method your client can use easily in their own country. Airwallex Invoicing lets you create and send invoices in 130+ currencies, so you can bill international clients easily.

Is sending an invoice the same as invoice processing?

No, sending an invoice and invoice processing are two different sides of the same transaction. Sending an invoice is what you do as the seller to request payment, while invoice processing describes how a business receives, checks, and pays invoices from its own suppliers. If you handle incoming supplier invoices too, our guide to invoice processing covers that side of the workflow.