Sending an invoice looks like a two-minute job: attach the PDF, write "please find attached", press send. It is also where most late payments actually begin — not because the client refuses to pay, but because the invoice arrived in a form their system could not process.
The five things the invoice itself must contain
Not "nice to have" — these are the fields that decide whether payment can be released:
- Your legal name and trading address, matching what the client's records expect. A mismatch with the contract is an automatic hold in most finance departments
- An invoice number, unique and sequential. Gaps raise questions during audits
- The issue date and the due date, both as dates. "Net 30" alone means the clock starts whenever someone decides it starts
- Line items with quantities and unit prices, and the tax breakdown separately from the net total
- A payment reference, usually the invoice number, plus bank details in a form that can be copied, not read off an image
If the invoice is going to a company rather than a person, add their purchase order number. Missing PO numbers are the single most common reason a large-company payment stalls.
Which format to attach
- PDF — the default for almost everyone. Print-proof, uneditable, works in every accounting system
- XML (UBL, ZUGFeRD, FatturaPA, Factur-X) — required or preferred by an increasing share of European public bodies and large buyers. If your client asks for one, sending a PDF instead costs you a month
- A photo of a paper invoice — only if the client asked for it. Otherwise it is the format most likely to be mistyped into their system
- A link to a hosted invoice — good for tracking whether it was opened, but never as the only delivery method
Rule of thumb: send the PDF, offer the structured file, never send only an image.
The email
Short, and it must pass what a finance assistant needs without opening the attachment:
Subject: Invoice 2026-114 — £4,000, due 11 November
Hi Anna,
Invoice 2026-114 for £4,000 is attached, issued today and due 11 November. PO number 4500219 is on the invoice.
Payment details are in the footer. Please quote the invoice number as the reference. If your system needs a different format — XML, or the invoice uploaded to your portal — tell me and I will send it today.
Marc
Four elements: the number, the amount, the due date, and an offer to fix format problems. That last sentence is what prevents the "we could not process it" email three weeks later.
Where invoices actually get stuck
Not in the client's inbox. In the queue:
- No PO number → returned to you, clock restarts
- Wrong entity name → sent to the client's legal department
- A missing tax ID (VAT number, EIN) → held for compliance
- Approval threshold — anything above a certain amount needs a second signature, and nobody chases that signatory for you
You can't fix their process. You can make sure your invoice has none of the four gaps.
Send it to the right person, once, properly
The person who ordered the work usually cannot release payment. Ask one question at the start of the engagement: "Who should invoices go to, and do you need a PO number?" — that single line saves the most common three-week delay.
Then send to that address every time, in the same thread. Invoices scattered across three email chains are the ones that get paid twice or not at all.
Then track it
Sending is not the end. Know three dates: when the invoice was sent, when it was due, and when the reminder goes out. If you cannot answer all three about your last five invoices, the problem is not your client — it's the absence of a process.
That is the part we automate: the invoice is read into data (number, date, amount, tax), the due date is set once, and the reminders leave on schedule from your own address. How invoice recognition works — or start with the reminder routine.