Invoicing & Billing System
Custom invoicing software with quotes, invoices and recurring billing, online payment links, automatic reminders, an aging report and a payment page customers can open without an account.
Project facts
- Project type
- Invoicing system
- Category
- Business systems
- Industry
- Services & finance operations
- Technology stack
- Next.js
- React
- TypeScript
- PostgreSQL
- Prisma
- BullMQ
- Stripe
Overview
This demonstration project shows an invoicing and billing system for a service business that sends a steady stream of invoices — one-off projects, monthly retainers and hours worked. It covers the full cycle: quote, invoice, reminder, payment and reporting, with VAT calculated correctly per line.
Customers receive a link to a clean payment page where they can pay by iDEAL or card, download the PDF and see earlier invoices. The business sees what is outstanding, what is overdue and how quickly customers pay.
The challenge
Invoices made in a word processor or spreadsheet take time, contain mistakes and are easy to forget to chase. Payments arrive by bank transfer with the wrong reference, recurring invoices are sent late, and nobody has a live view of outstanding money. Generic accounting tools help, but rarely fit a business's own numbering, templates, languages or approval steps.
The solution
The concept turns invoicing into a workflow. Quotes convert into invoices, recurring schedules create them automatically, and every invoice gets a payment link. Paid invoices are matched through the provider's webhook, so status updates itself; unpaid ones receive polite reminders on a schedule the business sets. Reports show outstanding and overdue totals, days to pay and VAT per period, ready for the accountant.
Key features
- Quotes that convert into invoices in one step
- Invoice editor with line items, VAT and live PDF preview
- Recurring invoices for retainers and subscriptions
- Online payment links with iDEAL, cards and wallets
- Automatic payment matching via webhooks
- Scheduled reminders for unpaid invoices
- Credit notes, partial payments and immutable numbering
- Aging report, days-to-pay and VAT summary
Who it is for
- Service businesses sending many invoices each month
- Agencies and consultancies with retainers
- Companies chasing late payments by hand
- Businesses that need invoices in more than one language
Technical approach
Next.js serves the admin and the public payment pages. Invoices, lines, customers and payments are stored in PostgreSQL via Prisma, with sequential numbering enforced in the database and issued invoices kept immutable — corrections are made with credit notes. PDFs are rendered on the server from the same templates as the web view. BullMQ runs recurring billing and reminders, and payments use hosted checkout with signed webhooks.
- Frontend
- Database
- Integrations
- Integrations
- MollieStripeiDEALSMTP
Screens
Customer-facing interface
Admin interface
Mobile & responsive views
Services behind this project
The work in this case study maps onto these services. Each page explains scope, process and what you receive.
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Questions about this type of project
Is this invoicing software in use by a real business?
No. It is a demonstration project with sample customers and invoices. It shows how Saad builds custom invoicing and billing software.
Does a custom invoicing system replace my accountant or accounting software?
Not necessarily. It handles day-to-day invoicing, payments and reminders; bookkeeping and tax returns can stay with your accountant. Exports or an API connection keep both in sync.
How do customers pay an invoice online?
Each invoice has a secure link to a payment page hosted by the payment provider, with methods such as iDEAL, cards or wallets. When the payment succeeds, the invoice is marked as paid automatically.
Want a similar project?
Tell Saad what you need. You get a clear scope, a phased plan and direct contact with the developer who builds it — no middlemen.
Direct, clear communication
You talk directly to the engineer writing the code — no sales intermediaries, no vague delivery dates.
Fully custom solutions
No ready-made WordPress themes or copied code — every line is written to fit your business's exact requirements and scale.
From idea to launch
Help shaping the specification and turning a rough concept into a real, usable digital product.
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