How to send an invoice so it gets paid

Start with who receives it

Most invoices go to the wrong person first. The client who hired you is rarely the one who approves payment, and at any company of size that person works a queue with rules. Universities publish theirs, and they are worth reading even if your client is a roofing supplier.

Duke wants emailed invoices as PDFs, ideally one per email, each with a valid purchase order number, and warns that a protected PDF fails to upload. Northwestern asks for one invoice attachment per email and no zip files, and says an invoice without its PO number should not go to accounts payable at all. Stanford will not process a PDF holding several invoices, and discards mailed invoices that lack a valid PO number.

The pattern holds for most clients: one invoice per file, their reference on it, no password. Gmail adds a reason to skip the zip, since it blocks some file types inside archives and password-protected archives entirely.

So ask on day one, in a single line: “Who should receive invoices, and do you need a PO number on them?” Then send to that address and copy the person who ordered the work, who can nudge accounts if it stalls. The PO number guide covers what to do when the number never arrives.

Be exact about names, too. The FTC's May 2026 alert warns small businesses that scammers send fake invoices and tells them to look up any company they do not recognize. An invoice from “DS Mechanical LLC” after a quote from “Dave's Plumbing” invites that search, so use the name and email the client saw on the quote.

When the client has a supplier portal

Some larger clients skip the inbox entirely. You register in their procurement system, and the invoice is a form you fill in there. MongoDB's published vendor instructions for Coupa are a fair example: you choose “Create Invoice from PO”, attach a copy of your invoice, and the invoice number, date and currency you type must match the attachment exactly. Payment terms must match the contract, and the line descriptions must match what you billed. A mismatch is a reason for the invoice to sit.

Some federal agencies take invoices through the Treasury's Invoice Processing Platform, which lets vendors submit invoices and follow their status through processing.

Two habits save weeks here. Register while the job is still running, because a portal account can take longer to approve than the work takes to finish. And ask whether they also want an emailed copy. An uninvited second copy gives their team two documents to reconcile for one bill.

References, remittance and tax details

Accounts payable matches your invoice to something on their side, so give them every hook: their PO number, your job number, and the invoice number as the payment reference, so the money arrives labeled. If you take bank transfers, put the account details at the foot of the invoice and in the email, and name the reference to quote.

A US business client that has to report what it pays you will ask for your taxpayer ID, and the IRS form for that is the W-9. Fill one in before your first invoice to a new company and attach it when they ask, so the paperwork is not what holds up the first payment.

Abroad, the invoice itself carries legal weight. In the UK, HMRC's VAT Notice 700 says a full VAT invoice must show your name, address and VAT registration number, the customer's name and address, and the VAT charged, and should normally be issued within 30 days of the tax point. In the EU, member states have been allowed since 14 April 2025 to introduce mandatory e-invoicing under specific conditions, according to the European Commission, so a client there may need a structured e-invoice through its own system rather than a PDF. Across a border, ask what format their tax rules require before you send.

Ways to send it, side by side

People treat this as a choice. It is usually a combination: a file for the record and a link for the action. Stripe's invoice email guidance recommends attaching the invoice and putting a payment link in the body.

My view on the editable file: send it only when asked. A spreadsheet that reflows in another version of Excel looks unfinished. Save to PDF and open it yourself before it goes.

A caveat on “opened”. Apple says Mail Privacy Protection downloads remote content in the background when a message arrives, and stops senders learning when and how often you view it. So an email “open” can be logged for a message nobody read. A link is recorded when someone loads the page, a better signal though still not proof of reading; the invoice tracking page shows what it records.

What goes in the email

Stripe's guide to invoice emails says to put the invoice number, a short description and the due date in the subject line, to keep it short because long ones get cut off on mobile, and to save “friendly reminder” for the follow-up. Name the file the same way, number first, so it can be found in a folder later.

The body has four jobs: what it is for and the amount, the due date as a date rather than “Net 30”, how to pay with the reference to quote, and a way to ask. Payment details belong in the message because the person paying often works from the email, not the filed attachment.

Make sure it arrives

Invoices ride on the same filtering rules as everything else. Google's sender guidelines require every sender to Gmail to set up SPF or DKIM for the sending domain and to keep the spam rate reported in Postmaster Tools below 0.3%, and it recommends staying below 0.1%. Microsoft requires domains that send more than 5,000 messages a day to Outlook.com, Hotmail and Live addresses to pass SPF and DKIM and publish DMARC, and routes mail that fails to Junk. You will not send 5,000 a day, but the direction is plain: mail from a domain with none of these records looks like what filters were built to catch.

In practice: send from your business domain, and ask whoever set up your email to confirm SPF and DKIM are in place. Keep the PDF small. Google's help page puts the personal Gmail sending limit at 25 MB and says a larger attachment is removed and replaced with a Drive link, which is not what an accounts inbox expects. If a message bounces, read the bounce: Gmail's help on returned mail lists an invalid address, spam filtering and a full inbox among the causes, and each needs a different fix.

When the client says they never got it

1. Check your side first. Find the message in your sent folder and look for a bounce. A mistyped address is the first thing to rule out.
2. Resend the same invoice. Same number, same date, same amounts, as a reply in the original thread so the first send date is visible. A new number creates a second invoice in their system.
3. Widen the net. Send it to the accounts address as well as your contact, and ask them to look in Junk or Spam and mark you as safe.
4. Ask about the portal. If they keep missing email, ask whether invoices should go through their supplier system instead.
5. Keep the record. Save the sent messages. In the UK, GOV.UK ties the default 30-day period to when the customer receives the invoice, so the date you can prove matters.

When to send it

The day the work ends, and if the job has a deposit, before it starts. The get paid faster guide covers why each day of delay is wasted. The send date also starts the clock: in the UK, when no payment date is agreed, payment is late 30 days after the customer gets the invoice or the work is delivered, whichever is later. An invoice on your computer starts nothing. For scale, Xero's US figures of 29 October 2026 put the average wait to be paid at 29.7 days in the September quarter, 8.6 days late; Xero does not define either measure, so read them loosely. I found no trustworthy study on the best weekday or hour to send, so be wary of anyone who quotes one.

Deposits and part payments

Send a deposit as its own invoice. The final one then shows the deposit as paid and the balance as due, the only number accounts needs. The deposit invoice guide has the wording, and progress billing covers stages. If you collect through a processor, check what it supports before you promise it: Square's help says payment schedules are not available together with late fees, and Stripe's says partial payments on its hosted invoice page must be switched on in settings. Say the split in the email: “Deposit of $640 due now, balance of $640 on completion.” If a client pays part of an unsplit invoice, do not rewrite it; record the payment and send the new balance with the original due date.

What makes clients delay, from the send

• It reached the person who hired you and nobody in accounts.
• It carries no reference their system recognizes, such as a PO number.
• It went by email when they only take invoices through a portal.
• The only way to pay is hidden in an attachment, or sits behind a login.
• It is a protected PDF, a zip file, or four invoices in one file.
• The sender name or address differs from the quote.
• It shows Net 30 but no due date.

If all of these are fixed and it is still stuck, the invoice approval debugger walks through where it is sitting on the client's side.

Follow-up timing, briefly

Stripe suggests a first reminder one to three days after the due date and a final notice a month or more after, with the invoice attached each time. With a new client or a large invoice I would add a two-line check a day or two after sending: “Did this reach the right person?” The payment reminder schedule works out the dates for your terms and has wording for each stage, and the late fee generator is for fees agreed up front.

Before you press send

• The recipient is the person, inbox or portal that approves payment, with the person who ordered the work copied.
• The sender name and address match the quote, and the email comes from your business domain.
• The subject line has the invoice number, a short description and the due date.
• The invoice is a PDF with no password, one invoice per file, well under 25 MB.
• The amount, due date, way to pay and payment reference are written in the email as well.
• The client's PO number or job reference is on the invoice.
• A new US company client has your W-9 if it asked; a UK VAT invoice shows your VAT number.
• You have opened the attachment and the payment link yourself.

How this works in InvoiceTools

You can build an invoice and download the PDF with no account, and nothing you type reaches our servers unless you sign in to save or send it. Turn on Pay now and every invoice gets a checkout run by Stripe, on every plan including Free. Reminders go out on a schedule you set, with your own wording on paid plans. The payment link generator builds a link and QR code for the provider you already use.

Common questions

Read next

Sources

• Duke University procurement, billing instructions for vendors
• Northwestern University procurement, submit invoices for payment
• Stanford Fingate, email or mail an invoice to accounts payable
• MongoDB, how to create an invoice in Coupa
• Bureau of the Fiscal Service, IPP vendor guidance
• IRS, about Form W-9
• HMRC, VAT Notice 700, section 16
• European Commission, VAT in the digital age
• Google, email sender guidelines, attachment limits and returned messages
• Microsoft, Outlook requirements for high-volume senders
• Gmail Help, file types you cannot send
• Federal Trade Commission consumer alert, 14 May 2026
• Stripe, tips for writing a great invoice email and partial payments
• Apple Support, protect email privacy in Mail
• Square Support, payments schedule
• GOV.UK, when a payment becomes late
• Xero Small Business Insights, United States