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How to Build an Invoicing App

Build your own invoicing app: clients, line items, tax, invoice numbers, PDFs, payment status and online payments, plus when to use invoicing software.

Mythex Team · 2026-09-29 · 7 min read

An invoicing app keeps a list of clients, lets you create invoices with line items and tax, gives each one a unique number, produces a PDF or a link the client can open, and tracks whether it has been paid. The first version is a straightforward database app you can build with AI in an afternoon or two. The details that matter — numbering, tax, locked invoices, payment status — are what separate a toy from something you can send to a client.

Decide what kind of invoicing tool you need

"Invoicing app" covers a few different things. Pick one before you start:

The third kind is where building your own pays off most: if you already record jobs or hours, generating the invoice from that data saves retyping. The fourth is a full SaaS MVP with its own concerns about accounts and billing.

The data model

Four tables cover most needs:

• Business settings — your name, address, tax or VAT number, logo, bank details, default payment terms, default tax rate, currency, and the next invoice number.
• Clients — name, company, email, billing address, their tax number if needed, and default terms.
• Invoices — number, client, issue date, due date, status, currency, notes, and totals (subtotal, tax, total).
• Line items — invoice, description, quantity, unit price, tax rate, and line total.

Add a payments table if clients pay in parts or by several methods: invoice, date, amount, method, reference.

The rules that matter

These are the parts AI builders will not get right unless you ask:

1. Sequential, unique numbers. Many tax systems expect invoice numbers to be unique and in sequence. Generate the number on the server when the invoice is issued, not when a draft is created, so abandoned drafts don't leave gaps.
2. Lock sent invoices. Once an invoice is sent, editing it silently is a problem. Allow edits on drafts only; for a sent invoice, void it and issue a new one, or issue a credit note.
3. Snapshot client details. If a client changes address next year, last year's invoices should still show the old one. Copy the client's details onto the invoice when it's issued.
4. Money as whole units of the smallest coin. Store amounts in cents (or pence), not as decimals, so totals don't drift from rounding. Round tax the way your tax authority expects, per line or per invoice.
5. Status that follows reality. Draft, sent, paid, partly paid, overdue, void. "Overdue" should be worked out from the due date, not set by hand.

PDF or invoice page?

Clients expect a PDF, but a web page is easier to keep in sync. Most invoicing tools offer both:

• A shareable invoice page with a hard-to-guess link, so the client can view and pay without logging in.
• A PDF download generated from the same data, with your logo and details.

Ask for a print-friendly layout and check the PDF on paper size (A4 or US Letter). Long line-item lists should break across pages cleanly.

Getting paid online

Showing bank details on the invoice is enough for many businesses. If you want card payments, connect a payment provider:

• Stripe Checkout or a payment link for the invoice amount. The client clicks "Pay now" on the invoice page.
• A webhook from the provider tells your app when the payment succeeds, and the app marks the invoice paid. Don't mark it paid just because the client returned to a success page — that page can be reached without paying. See how to add payments to your app.
• Test mode first. Use the provider's test keys and test cards until the whole flow works.

Worth knowing: Stripe also offers its own invoicing product with hosted invoice pages, as of September 2026. If all you need is to send an invoice and get paid, that may be enough on its own.

Reminders

Automatic "your invoice is due" emails are one of the most valued features and one of the easiest to get wrong. You need:

• An email provider (Resend, Postmark, SendGrid and similar) — see sending emails from your app.
• Something that runs on a schedule to find overdue invoices. A published web app that sleeps when idle won't wake up by itself, so ask the builder how the check will be triggered — for example a scheduled call from an external cron service to an endpoint in your app.
• A record of which reminders were sent, so nobody gets the same email twice.

Start with a manual "Send reminder" button. Automate later.

A first prompt that works

Build an invoicing app for my web design business. I log in and manage clients (name, company, email, billing address, VAT number). I create invoices with line items (description, quantity, unit price), a VAT rate per line, and see subtotal, VAT and total calculated live. Invoice numbers are sequential (INV-0001, INV-0002) and assigned when I mark an invoice as sent; drafts stay editable, sent invoices are locked. Each invoice has a status (draft, sent, paid, overdue) and a due date; overdue is worked out from the due date. Each invoice gets a private shareable link and a PDF download with my logo and bank details. Dashboard shows total outstanding and overdue. Store amounts in pence. GBP only. Save everything in a database.

Build steps

1. Settings and clients first. Get your business details and client list working, since every invoice depends on them.
2. Invoice editor. Line items, tax and totals. Test with awkward numbers — 3 × £33.33 at 20% VAT — and check the totals against a calculator.
3. Numbering and locking. Issue three invoices, delete a draft, issue another. Numbers should stay in sequence with no gaps or duplicates.
4. Invoice page and PDF. Open the share link in a private window. Print the PDF.
5. Payments. Add Stripe in test mode, pay an invoice with a test card, confirm the webhook marks it paid.
6. Security check. Log out and try to open another client's invoice by changing the ID in the URL. Only the private link should work, and it shouldn't reveal other invoices. Our security checklist has more to try.
7. Export. Add a CSV export of invoices and payments for your accountant.

Common mistakes

• Floating-point money. £0.1 + £0.2 not equalling £0.3 is a real bug in real apps. Store whole minor units.
• Editable sent invoices. Changing an invoice after the client has it causes disputes and accounting problems.
• Numbers assigned on draft. Creates gaps and confusion.
• Guessable invoice links. /invoice/42 invites someone to try /invoice/43. Use long random tokens.
• Assuming one tax rule. Tax rates, reverse charge, and required wording differ by country and customer type. Ask an accountant; don't let the AI guess.
• No backup. Invoices are records you may be required to keep for years. Export regularly, and on plans that allow it, keep the code in GitHub.

Costs and tools

When ready-made invoicing software is better

Build your own when invoicing is part of your own workflow, when you want a tool shaped exactly around how you bill, or when you're adding invoices to an app you already have. Choose established accounting or invoicing software when:

• You need bookkeeping, bank feeds, expense tracking and tax returns in one place.
• Your country requires e-invoicing in a specific format or through a certified channel.
• You'd rather your accountant work in a tool they already know.

A common middle path: build the part that is specific to you (jobs, timesheets, a client portal where clients see their invoices) and export the numbers to your accounting software. Build vs buy software for small businesses walks through the decision.

Building it with Mythex

On Mythex you describe the invoicing app in chat and watch it take shape in a live preview. It adds a database when the app needs one, keeps API keys such as your Stripe secret in project secrets, and publishes the app to a URL you can use straight away. The Stripe recipe in the docs shows the bring-your-own-keys setup. To start from a prompt that already describes the basics, try the invoice generator or freelance invoicing templates.