How to send invoices online: a step-by-step guide

Sending an invoice looks trivial until a client bounces it back because a field is missing, or until your accountant can't reconcile payments because two invoices share the same number. This guide covers the full process, from mandatory fields to chasing the payment.

What an invoice must include

Exact requirements vary by country and tax regime, but one set of fields shows up in practically every jurisdiction. Include them and your invoice is very unlikely to be rejected:

• The word "Invoice" clearly visible, so it isn't mistaken for a quote or a receipt.
• A unique, sequential invoice number. No duplicates, no strange gaps: it's the first thing an audit looks at.
• Issue date and, where relevant, the date the service was delivered.
• Your full details: legal or business name, address, tax ID (EIN, VAT number, or local equivalent) and contact information.
• Client details: legal name, address, and tax ID where the law requires it.
• A line-item description, with quantity, unit price, and amount. A bare "professional services" line is an invitation to argue about the total.
• Subtotal, itemized taxes, and total. Tax always shows separately, never baked into the price.
• Payment terms: due date (net 15, net 30), accepted methods, and bank details or a payment link.

If you bill clients abroad, state the currency explicitly. "1,500" doesn't mean the same thing to someone invoicing in dollars as to someone receiving in another currency.

How to number invoices without getting it wrong

Numbering is the most common mistake among freelancers who invoice by hand. The rule is simple: sequential, no gaps, never reuse a number. A scheme that works well is YYYY-NNN (for example 2026-001, 2026-002), resetting the counter each fiscal year.

If you void an invoice, don't delete the number: issue a credit note that references it. Deleting leaves a hole in the sequence, and that hole is exactly what triggers awkward questions during a review.

Step by step: sending an invoice online

1. Save the client once. Store their legal name, address, tax ID, and billing email. Retyping those details on every invoice is the main source of typos.
2. Create the invoice and let the number assign itself. Any serious tool generates the sequence for you.
3. Add detailed line items. One line per deliverable, with hours or quantities whenever the work is billed per unit.
4. Set taxes and payment terms. Define the due date explicitly; "as soon as possible" is not a payment term.
5. Check the total and the tax breakdown before sending. A tax error caught afterwards forces you to issue a credit note.
6. Send by email with the PDF attached and a payment link. The less friction the client faces, the sooner you get paid.
7. Record the payment when it lands and mark the invoice as paid, so it drops out of your reminders.

PDF, email, or payment link: which format to use

PDF is still the standard file format: it's immutable, it prints identically everywhere, and accounts payable departments expect it. Always attach it.

The payment link is what actually speeds up collection. An invoice that forces the client to open their bank, copy an account number, and type a reference gets paid days later than one with a payment button.

Email is the channel, and it's worth handling with care: subject line with the invoice number and your name, short body stating the amount and the due date. Skip the bare "attached as agreed."

How to follow up until you get paid

Most late payment isn't bad faith, it's forgetfulness. A simple reminder schedule solves nearly all of it:

• On issue: confirmation of receipt.
• 3 days before the due date: friendly reminder.
• On the due date: notice that the invoice is due today.
• 7 and 14 days past due: follow-up with the outstanding amount.

Automating these notices beats sending them by hand, because then the awkward reminder comes from the system rather than from you.

Use a tool, not a template

A spreadsheet template works for your first ten invoices. After that the problems start: duplicate numbering, totals that don't add up, no visibility into what's still outstanding, and no history per client.

SIM is Startit's invoicing app for freelancers and small businesses: it creates professional invoices with automatic numbering, emails them with a payment link, and shows you issued, paid, and overdue amounts on a single screen. If you're invoicing from a spreadsheet today, it's the first change your cash flow will notice.