How to Build an Invoice Tool as a Solo Developer

What You're Building

An invoice tool lets freelancers and small businesses create, send, and track invoices. It sounds boring, and that's exactly why it's a great business. Every freelancer, consultant, and small business owner needs to send invoices, and a surprising number of them are still using Word documents or Google Docs templates. The pain is real and recurring.

I send invoices regularly and I've used everything from FreshBooks to literally just emailing a PDF I made in Google Docs. The tools that won me over were always the simplest ones. Create invoice, send it, get paid, track it. No bloat. No 47-tab dashboard. Just invoicing that works.

Difficulty & Timeline

Recommended Tech Stack

Next.js or SvelteKit for the dashboard, with a Node.js or Django backend. For PDF generation, use a library like Puppeteer (render HTML to PDF) or a service like Gotenberg. Don't try to build PDF generation from scratch with low-level libraries. HTML-to-PDF gives you beautiful, customizable invoices with minimal effort.

PostgreSQL for storing clients, invoices, line items, and payment status. Stripe for accepting payments on behalf of your users (Stripe Connect if users want to accept card payments through your platform) or simply integrate with payment tracking where users manually mark invoices as paid.

Step-by-Step Plan

Phase 1: Core Invoicing (Week 1-3)

Build the invoice creator. Users add their business details, client information, line items (description, quantity, rate), tax rates, and notes. Calculate totals automatically. Show a real-time preview of the invoice as they build it.

Add PDF generation. When the invoice is ready, generate a clean, professional PDF. Use an HTML template styled with CSS, render it with Puppeteer or a similar tool. This approach lets you create beautiful invoices without fighting PDF libraries.

Build the client management system. Users add clients once and reuse them across invoices. Store client name, email, address, and any default payment terms.

Phase 2: Sending & Tracking (Week 3-5)

Add email delivery. Users click "Send" and the invoice gets emailed to the client as a PDF attachment (plus an optional web link to view and pay online). Use Resend or SendGrid for reliable email delivery.

Build the dashboard showing all invoices with their status: draft, sent, viewed, paid, overdue. Automatic overdue detection based on due dates. Optional reminder emails for overdue invoices since this is a feature freelancers love because nobody enjoys chasing payments manually.

Add a public invoice page where clients can view the invoice in their browser and optionally pay online. This web view should be clean, professional, and mobile-friendly.

Phase 3: Payments & Polish (Week 5-6)

Integrate online payments. Let your users accept credit card or bank transfer payments directly through the invoice. Stripe Connect is the cleanest option here. When a client pays, the invoice automatically updates to "Paid" status.

Add recurring invoices for users with subscription clients. Monthly retainers are common for freelancers and consultants. Set it once, invoices go out automatically.

Build a simple reporting dashboard. Total invoiced, total paid, total outstanding, revenue by month. Freelancers need this data for tax time and it takes maybe a day to build.

Key Features to Build First

Invoice creator with live preview. Line items, tax calculation, customizable templates. The preview should look exactly like the final PDF.

PDF generation. Clean, professional PDFs that users aren't embarrassed to send. Use HTML-to-PDF for easy customization.

Email delivery. One-click send with the PDF attached. Track when the client opens the email if possible.

Status tracking. Draft, sent, viewed, paid, overdue. Automatic overdue detection with optional reminders.

Client management. Store client details for reuse. Saves time on every subsequent invoice.

Architecture Overview

Dashboard (Next.js / SvelteKit)
  └── API (Node.js / Django)
        ├── Invoice CRUD (with line items)
        ├── Client management
        ├── PDF generation (Puppeteer / Gotenberg)
        ├── Email delivery (Resend / SendGrid)
        ├── Payment processing (Stripe Connect)
        ├── Recurring invoice scheduler
        └── Billing (your plans via Stripe)

Storage
  ├── PostgreSQL (invoices, clients, users)
  ├── S3/R2 (generated PDFs, logos)
  └── Redis (job queue for emails, reminders)

Common Pitfalls

Making ugly PDFs. The invoice PDF is your product's first impression. If it looks like a spreadsheet dump, nobody will take it seriously. Invest time in a clean, professional template. Look at what FreshBooks and Wave invoices look like and match that quality.

Ignoring tax complexity. Different countries have different tax requirements. VAT, GST, sales tax, reverse charge. You don't need to support all of them at launch, but design your data model to accommodate multiple tax rates and tax types. At minimum, support a configurable tax percentage per invoice.

Building an accounting system. You're building an invoicing tool, not QuickBooks. Don't add expense tracking, bank reconciliation, or double-entry bookkeeping. Stay focused on creating, sending, and tracking invoices. Users who need full accounting will connect your tool to their existing accounting software.

Overcomplicating templates. Start with one beautiful, professional template. You can add more later. I've seen invoice tools launch with 20 mediocre templates instead of 3 excellent ones. Quality over quantity.

Not handling currencies properly. Freelancers work internationally. Support multiple currencies from day one. Store amounts in the invoice's currency, not your default currency. Display the currency symbol correctly. This is easy to design for upfront and painful to retrofit.

Timeline Estimate

Is This Worth Building?

Absolutely. Invoicing is one of those evergreen businesses that will always have demand. Every freelancer, consultant, and small business needs it. The competition (FreshBooks, Wave, Zoho Invoice) is established but often bloated and expensive. There's real space for a simpler, cheaper alternative that nails the core workflow.

The freemium model works great here. Let users send 5 invoices/month for free, charge $9-19/month for unlimited invoices, custom branding, and recurring invoices. A $19/month price point is easy for any freelancer to justify when they're invoicing thousands per month. 500 paying users at $15/month is $90k/year. And invoicing tools have very low churn because switching means migrating client data and invoice history.