The Problem Every freelancer needs invoices. Most "free" tools aren't really free — they limit you to 5 invoices/month, require an account, or add watermarks. I wanted to build the simplest possible invoice tool with zero ongoing costs. The Architecture The entire application runs client-side in the browser: No backend — all logic runs in JavaScript No database — form state lives in the DOM No auth — no accounts to manage No server costs — Firebase Hosting free tier handles everything Key Technical Decisions PDF Generation: I used jsPDF to generate invoices client-side. The PDF is created entirely in the browser and downloaded directly — no server round-trip needed. Offline Support: The app is a Progressive Web App (PWA) with a service worker. Once loaded, it works without an internet connection. Privacy: Since there's no backend, user data never leaves the browser. No cookies, no tracking beyond basic Firebase Analytics. SEO Strategy Instead of building one page and hoping for traffic, I created dedicated landing pages for each target audience: /freelancer-invoice-template.html /contractor-invoice-template.html /consulting-invoice-template.html /small-business-invoice-template.html /how-to-create-an-invoice.html Each page targets specific long-tail keywords and includes FAQ schema markup for rich search results. Monetization The core tool is free forever. Revenue comes from a premium template pack sold on Payhip for 9.Thefreetooldrivesorganictraffic;thetemplatesconvertasmallpercentage.ResultsRunningcost:9. The free tool drives organic traffic; the templates convert a small percentage. Results Running cost: 0/month Deployed in under a day 6 pages indexed, targeting multiple invoice-related keywords Try It Check it out at https://invoicefreely.web.app Feedback welcome — what would you add or change?