Learn how to send an invoice to a client correctly — what to check before sending, how to deliver it, and how to follow up to get paid on time.
Completing the work is the hard part. Sending the invoice should be simple — but it's where a surprising number of payments get delayed. The wrong contact, a missing PO number, an unclear subject line, or no follow-up process can add weeks to your payment cycle before the job even gets into your client's payment queue.
This guide covers how to send an invoice to a client correctly: what to check before it goes out, how to deliver it, what to write in the email, how to handle different client types, and what to do when payment doesn't arrive on time.
Before You Send: A Pre-Send Checklist
Sending an invoice with an error is worse than sending it late. Errors create queries, queries create delays, and delays push your payment further back in your client's queue. Run through this checklist before every invoice goes out.
Confirm the Invoicing Contact
Your day-to-day project contact is often not the person who processes invoices. Many businesses — particularly larger ones — have a dedicated accounts payable contact or a specific invoicing email address (accounts@, finance@, ap@). Ask at the start of a new client relationship: "Where should I send invoices, and is there a specific contact or reference I need to include?" Sending to the wrong address is one of the most common reasons invoices go unpaid for weeks with no one realising.
Get the Purchase Order Number (If Required)
Many businesses won't process an invoice without a matching purchase order (PO) number. If your client uses a PO system, get the number before work begins — not after the invoice is ready. An invoice that arrives without a valid PO will be returned or queued for investigation before it's even looked at.
Check the Invoice Itself
Before sending, confirm:
- The invoice number is unique and sequential
- The client name and billing address are correct
- Line item descriptions are specific and match what was agreed (see how to write an invoice properly)
- The total is correct, with tax shown separately if applicable
- Your payment terms and due date are clearly stated
- Your bank details or payment link are included
- Your VAT, GST, or GST/HST number is present if you're tax-registered
Confirm Your Payment Terms Are Agreed
If this is your first invoice with a new client, make sure your payment terms aren't a surprise. Net 30 when they were expecting Net 60 — or vice versa — can trigger a dispute before the invoice is even processed. Payment terms should be agreed before work starts, ideally in writing.
How to Deliver Your Invoice
Email (Most Common)
Email is the standard delivery method for invoices in the US, UK, Australia, and Canada. Send your invoice as a PDF attachment — it's not easily editable, it renders consistently across all devices, and most accounting software can read and process it automatically. Name the PDF file clearly (e.g. InstantInvoice-INV-047-ClientName.pdf) rather than using a generic filename. Use a clear subject line and keep the email body brief and professional.
Online Invoicing Link
Many invoicing platforms send a payment link rather than a PDF attachment. The client clicks the link, views the invoice in a browser, and can pay directly. This approach tends to produce faster payment because the friction between opening the invoice and paying it is minimal. It also gives you read receipts and payment tracking without any manual effort.
Post (Rare but Sometimes Required)
Some clients — particularly in construction, local government, and certain professional services — still require a paper invoice by post. If this is the case, keep a copy for your own records, note the postage date, and follow up by email to confirm receipt.
Accounting Software to Accounting Software
Larger clients sometimes use e-invoicing systems that receive invoices directly into their accounting platform. In Australia, the ATO has been actively promoting Peppol e-invoicing for B2B transactions. If a client requests this, ask for their specific requirements — it's more technical than standard email delivery but significantly faster to process on their end.
What to Write in Your Invoice Email
The email that carries your invoice is not just a wrapper — it's the first thing your client reads. A clear, professional email sets the tone and reduces the chance of queries.
Subject Line
Be specific. Generic subject lines get buried.
| Type | Example |
|---|---|
| Weak | Invoice attached |
| Weak | November work |
| Strong | Invoice INV-047 — Brand Strategy Project — Due 14 February 2026 |
Include your invoice number, a brief project reference, and the due date. This makes the email searchable and tells the recipient immediately what they're looking at.
Email Body
Keep it short. The invoice contains all the detail — the email just needs to confirm what's attached and make it easy to act. Here's a template that works:
Subject: Invoice INV-047 — Brand Strategy Project — Due 14 February 2026
Hi [Name],
Please find attached Invoice INV-047 for the brand strategy project, totalling £2,400 (due 14 February 2026). Payment details are included on the invoice. Please don't hesitate to get in touch if you have any questions.
Best regards, [Your name]
What Not to Write
- Don't apologise for sending the invoice
- Don't be vague about the amount or due date
- Don't bury the invoice number in a wall of text
- Don't use "per our conversation" as a substitute for clear line items on the invoice itself
Invoicing Different Types of Clients
The core process is the same, but different client types have different expectations and requirements.
| Client Type | Key Considerations |
|---|---|
| Individual / Consumer | No PO system, no VAT reclaim needed; focus on clear description, due date, and easy payment method |
| Small Business | May be VAT/GST-registered; confirm upfront to avoid reissuing documents later |
| Large Corporate | Formal PO process, accounts payable team, approval workflows of 7–14 days, possible supplier portals |
| International Client | State currency clearly, confirm tax treatment on exports, account for SWIFT fees and transfer timing |
Individual / Consumer Clients
Invoicing a private individual is typically the simplest scenario. They don't need a VAT invoice for tax purposes, they rarely use PO systems, and they're usually comfortable paying by bank transfer or card. The main things to get right: a clear description of what you did, a specific due date, and easy payment instructions.
Small Business Clients
Small businesses often handle their own bookkeeping, so a clean, clearly formatted invoice makes their life easier and speeds up payment. If they're VAT or GST-registered, they'll need a compliant tax invoice. Confirm this upfront so you're not reissuing documents later.
Large Corporate Clients
Large organisations typically have a formal purchase order process, an accounts payable team separate from the project team, and invoice approval workflows that can take 7–14 days even when everything is correct. Some large clients have a supplier portal where invoices must be uploaded — emailing the project manager won't work. Find out their process before you invoice for the first time.
International Clients
When invoicing across borders, state the invoice currency clearly and specify who bears the cost of currency conversion. You generally don't charge your domestic VAT or GST to overseas clients — exports are typically zero-rated, but confirm this with an accountant if you're unsure. International bank transfers can take 2–5 business days, so factor this into your payment terms.
How to follow up on an unpaid invoice
Most late payments aren't intentional. Invoices get buried, payment runs get missed, and approvals get delayed. A structured follow-up process — applied consistently and without apology — resolves the majority of overdue invoices without damaging client relationships.
| Timing | Action | Tone |
|---|---|---|
| Day 0 (due date) | First reminder — confirm invoice received, ask if anything is holding up payment | Friendly |
| Day 7 (one week overdue) | Second reminder — reattach invoice, ask for expected payment date | Professional, slightly firmer |
| Day 14 (two weeks overdue) | Formal notice — reference payment terms, note potential late payment charges | Direct and formal |
| Day 21+ | Escalation — formal letter before action, debt collection, or statutory claim | Formal |
Day 0 (Due Date): First Reminder
If payment hasn't arrived by the due date, send a brief, friendly reminder the same day. Don't wait a week to give them time — the due date is the due date.
Day 7 (One Week Overdue): Second Reminder
Slightly firmer in tone, but still professional. Reattach the invoice and ask for a confirmed payment date.
Day 14 (Two Weeks Overdue): Formal Notice
Be direct. State that the invoice is overdue, reference your payment terms and the original due date, and note that late payment interest may apply if your terms include it.
Day 21 and Beyond: Escalation
If three reminders have produced no payment and no communication, escalate. Options include a formal letter before action, referral to a debt collection agency, or — for UK businesses — a claim under the Late Payment of Commercial Debts Act. Keep records of every invoice sent, every reminder, and every response.
Invoicing Mistakes That Delay Payment
Invoicing Late
Every day between completing work and sending the invoice is a day added to the end of your payment window. Invoice the moment work is delivered. If you're on a retainer or recurring billing arrangement, invoice on the same date every month — predictability helps clients plan their payment runs.
No Confirmation of Receipt
For high-value invoices, follow up briefly to confirm the invoice was received and is being processed. A quick check-in two days after sending can save three weeks of waiting.
Accepting "It's in the System"
When a client tells you an invoice is in their system but you haven't been given a payment date, ask for one. "In the system" is not a payment commitment. Politely ask: "Can you confirm an expected payment date?" If they can't, it may not be in the system at all.
Sending a Proforma Instead of a Final Invoice
A proforma invoice is a preliminary document. Some clients will pay it, but it's not a valid tax invoice and neither party has a compliant financial record until a proper invoice follows. If you've been using proformas as final invoices, correct this going forward.
No Written Payment Terms
If you never stated your payment terms — in a contract, a proposal, or on the invoice itself — enforcing them becomes difficult. State your terms clearly before work starts and repeat them on every invoice.
Frequently Asked Questions
When should I send an invoice to a client?
Send it as soon as the work is complete — or at the agreed billing milestone. For project work, that typically means the day of delivery. For retainers and recurring services, invoice on a fixed date each month. The longer you wait, the longer you'll wait to be paid.
Should I call or email to follow up on an overdue invoice?
Both have their place. Email is better for creating a paper trail and giving the client a chance to respond in their own time. A phone call is better when emails have gone unanswered for more than a week or when you need a payment commitment quickly. Use email first, escalate to a call if there's no response within a few days.
What format should I use to send an invoice?
PDF is the standard. It's not editable, renders consistently across all devices, and is accepted by accounting software for automatic processing. Name the file clearly — include your business name, the invoice number, and the client name.
Do I need to send a receipt after payment?
Strictly speaking, an invoice and a bank statement together serve as a payment record. But issuing a receipt is professional and appreciated — it closes the loop for the client and confirms the transaction is complete on your end. Some clients request receipts for their own bookkeeping.
Can I charge interest on a late invoice?
In most jurisdictions, yes — if you've stated this in your payment terms upfront. In the UK, the Late Payment of Commercial Debts Act allows businesses to charge statutory interest on overdue B2B invoices. In Australia, Canada, and the US, late payment interest is enforceable where it has been agreed in writing before the work began. The right to charge it must be communicated in advance.
Key Takeaways
- Confirm the correct invoicing contact and any PO number requirements before you send the first invoice to a new client. The wrong address is one of the most common causes of preventable payment delays.
- Use a clear, specific subject line that includes the invoice number, project reference, and due date. Generic subject lines get buried.
- Keep the covering email brief and professional. The invoice is the document — the email just needs to confirm what's attached and make it easy to act.
- Send invoices promptly. Every day you delay is a day added to the end of your payment window.
- Build a structured follow-up process and apply it consistently — a reminder on the due date, a firmer nudge at 7 days, a formal notice at 14 days. Most late payments resolve at the first or second reminder.