Case Study: Billing Automation for a SaaS Startup
Eliminating manual invoicing with a lightweight AI-assisted solution
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The Challenge
Our client, a growing B2B SaaS startup with steady MRR, faced a recurring bottleneck in billing operations. Their customer-specific pricing terms made billing highly customized and difficult to standardize.
Each month, the financial controller had to:
- Export sensitive data from the database (kept behind a firewall for compliance reasons).
- Build pivot tables and apply formulas manually.
- Generate invoices in Excel, validate each one, and send them to customers.
Existing invoicing platforms were too rigid to adapt to these complex billing rules, and the development team was fully occupied with the core product. As a result, automated invoicing was never prioritized, leaving the finance team burdened with repetitive, error-prone work.
The Solution
We designed and implemented a lightweight internal billing tool - custom-built to the startup’s needs while keeping compliance and simplicity at the forefront.
Built as a Next.js application deployed on Vercel, the tool has a simple UI and a few endpoint - yet it fully replaced a previously manual, multi-hour monthly workflow.
Secure input handling
The tool accepts CSV data stripped of PII, pulled directly from the database for this specific billing task.
Custom rule application
Billing logic such as variable costs, customer-specific terms, and currency conversion was implemented to replicate the manual process.
Automated outputs
Using ExcelJS, jsPDF, and JSZip, the app generates invoices, client-specific reports, and aggregated summaries.
Access control
Integrated with Stytch authentication, allowing only the financial controller to log in and use the software.
Stateless design
All calculations are done in-flight, with no persistent database storage, reducing compliance risks.
The Results
Time savings
Billing is now completed practically instantly, with the entire monthly cycle reduced to just a few clicks.
Controller workload reduced
The financial controller now spends less than 8 hours per month in total on billing and reconciliation.
Error reduction
Automated calculations eliminate formula mistakes and inconsistencies.
Cost efficiency
The tool was built in less than 16 hours and for under $1,000, compared to the cost of onboarding new SaaS billing platforms.
Scalability
The repository is structured with AI-assisted documentation, making it simple to add new clients or billing rules with minimal effort.
Conclusion
This project highlights how AI and lightweight modern frameworks are changing the economics of software development in 2025. Instead of relying on rigid SaaS billing tools or overloading product teams, startups can quickly build fit-for-purpose internal apps that deliver immediate business value.
For our client, this meant:
- Cutting billing time to a fraction.
- Automating invoices so they are generated practically instantly.
- Reducing reliance on third-party SaaS tools.
The billing tool now serves as a foundation for future automation, with plans to extend it for reconciliation and reporting. What once required days of manual effort is now a streamlined process managed by a simple internal app.
Tech Stack
- Framework: Next.js, deployed to Vercel
- Authentication: Stytch
- Data processing: In-memory calculations (no persistent DB)
- Exports: ExcelJS, jsPDF, JSZip
- AI-assisted development: Cursor with modern models to accelerate setup and code generation
Time & Budget
- Requirements gathering: < 4 hours
- Implementation: < 8 hours
- Total effort: < 16 hours
- Total cost: < $1,000