I need a full-featured billing solution that runs natively on Windows and is complemented by mobile apps for iOS and Android. Core modules must include invoice generation, expense tracking, payment processing, and integrated barcode scanning so products can be added with a quick scan instead of manual entry. A secure, role-based user-authentication layer is mandatory. All clients—desktop and mobile—should share one cloud-hosted database, keeping records in sync even when devices work offline and later reconnect. I’m open to the tech stack, but please specify the frameworks and tools you propose (for example .NET/C# for Windows, Flutter or React Native for the mobile apps, and SQL Server or PostgreSQL on the back end) as well as the payment gateway you recommend. Deliverables • Windows installer with auto-update • iOS and Android builds ready for store submission • Complete, well-commented source code and build instructions • Brief user manual • 30-day bug-fix support Acceptance criteria 1. Create, edit, and email a PDF invoice from either platform in under three clicks. 2. Expenses recorded on mobile show up on the desktop app within 10 seconds. 3. Barcode scans instantly populate product fields. 4. Successful payment updates invoice status automatically and logs the transaction. 5. Login enforces hashed passwords and locks out after repeated failures. When you respond, outline your proposed stack, timeline, and any similar projects you’ve shipped.