I Built a Free, Offline Invoice App with Flutter

Posted on March 13, 2026  •  5 minutes  • 979 words

Every time I needed to invoice a client, I ran into the same wall: every decent invoice tool either locks your data in the cloud, wants a monthly subscription, or both. I just wanted something that works offline, keeps my data on my machine, and doesn’t cost me anything.

So I built it.

Invoiso is a free, open-source desktop invoice app for Windows and Linux. No account. No internet. No recurring fee. Just a binary you run on your machine.

Why Flutter for a Desktop App?

Flutter is usually talked about in the context of mobile. But Flutter’s desktop support has matured a lot — and it gave me a few things I really wanted:

• One codebase for Windows and Linux — I didn’t want to maintain separate projects.
• A rich widget ecosystem — building forms, data tables, and PDF previews is fast.
• Dart’s type safety — for a data-heavy app with models, migrations, and services, it helps.

The build output is a native binary — no Electron, no Chromium, no 300 MB runtime. The Linux AppImage is lean and starts instantly.

The Stack

SQLite with sqflite_common_ffi is the key piece for desktop. The mobile sqflite package doesn’t work on Linux/Windows — sqflite_common_ffi wraps the native SQLite library and exposes the same API, so the transition is seamless. All data lives in a single .db file the user controls.

Project Structure

lib/
├── main.dart
├── common.dart          # Shared enums and data classes
├── constants.dart       # UI constants
├── database/            # SQLite helpers and CRUD services
├── models/              # Invoice, Customer, Product, ...
├── providers/           # Riverpod state providers
├── screens/             # All UI screens
├── services/            # PDF generation, CSV/PDF export
├── backup/              # Backup and restore logic
└── utils/               # Logger, formatters, error handler

The database layer follows a service pattern — each entity (invoices, customers, products) has its own service class wrapping raw SQL. Migrations are handled manually in database_helper.dart with a version counter.

Interesting Technical Bits

1. PDF Generation with Three Templates

PDF output is generated using the pdf package (not a web renderer — actual vector PDF). I built three templates: Classic, Modern, and Minimal — each with a distinct layout and color scheme.

The tricky part was making the layout adaptive to variable-length item lists, long product descriptions, and optional sections (GST, UPI QR, logo). The pdf package has its own layout model (similar to Flutter widgets but without hot reload — so a lot of trial and error).

2. UPI Payment QR Code on Invoices

For Indian users, I added UPI QR code embedding. When a UPI ID is configured in settings, the PDF generation encodes a upi://pay?pa={upi_id}&am={total} URI into a QR code using the qr package and renders it directly in the PDF footer. Clients can scan it with GPay, PhonePe, or Paytm to pay instantly.

3. Multi-Currency with Per-Invoice Snapshots

Currency is selected globally in settings, but each invoice stores its own currency_code and currency_symbol at creation time. This means if you change your default currency later, old invoices still display correctly. A simple but important detail — got it wrong the first time and had to add a DB migration.

Speaking of migrations: I maintain a DB_VERSION constant and a chain of _upgradeDbV{n} functions. Each upgrade is additive (new columns with defaults, new tables) — no destructive changes.

4. Backup and Restore

The backup feature is a plain file copy of the SQLite .db file to a user-chosen directory. Restore reverses it. Simple, robust, and the user owns the backup file — they can put it on a USB drive, copy it to another machine, or commit it to their own repo.

Features at a Glance

• PDF invoices — one-click generation in Classic, Modern, or Minimal templates
• Invoices and Quotations — with color-coded tracking
• Invoice cloning — duplicate any document with one click
• Bulk actions — multi-select to export CSV, generate PDFs, or delete
• UPI QR code — embedded scannable payment QR in every PDF
• GST ready — GSTIN fields, HSN codes, per-item or global tax rate
• Multi-currency — INR, USD, EUR, GBP, JPY, AED, SGD, AUD, CAD, JMD
• Customer and product management — full CRUD with search and pagination
• Soft delete / Trash — recoverable deleted invoices
• CSV export — for spreadsheets or accounting software
• Backup and restore — one-click, local file
• Security — login with brute-force protection and session timeout
• 100% offline — zero network calls, ever

Download

Grab the latest from the GitHub Releases page .

Build from Source

git clone https://github.com/Anooppandikashala/invoiso.git
cd invoiso
flutter pub get

# Run in debug mode
flutter run -d linux    # or -d windows

# Release build
flutter build linux --release
flutter build windows --release

Prerequisites: Flutter SDK >= 3.3.3. On Linux you’ll also need clang, cmake, ninja-build, and libgtk-3-dev.

What’s Next

There’s a solid list of planned features:

• Invoice status tracking (Paid / Unpaid / Overdue / Draft) — the biggest gap right now
• Due dates and payment terms (Net 7, Net 30, etc.)
• Payment recording with partial payment support
• Recurring invoices for retainers and subscriptions
• Revenue reports with date range filters
• Dark mode
• Global search across invoices, customers, and products
• GST report export in GSTR-1 compatible format (for Indian users)

If any of these interests you, contributions are welcome.

Contributing

1. Fork the repo
2. Create a branch: git checkout -b feature/your-feature
3. Open a pull request

Bug reports and feature requests go in GitHub Issues .

The whole thing is MIT licensed. If it saves you time or money, great — that’s the point. If you want to help make it better, even better.

GitHub: github.com/Anooppandikashala/invoiso Website: anooppandikashala.github.io/invoisoapp