How to Build a Browser Tool and Sell It on Gumroad — A Complete Guide

I built 24 browser-based tools. Here is the complete technical guide for building your own and selling PRO licenses on Gumroad. Architecture: One HTML File + GitHub Pages + Gumroad No frameworks, no backend, no monthly costs. One HTML file on GitHub Pages. Gumroad handles payments. The PRO Flow User hits free limit → PRO modal Buy button → Gumroad popup ( window.open , NOT target=_blank ) Payment → Gumroad postMessage with license key message listener catches key Key verified via Gumroad API ( /v2/licenses/verify ) localStorage saves PRO (in try-catch!) Gotchas From 24 Tools postMessage security: Check d.success && d.purchase , never just d.license_key . I had this bug in 17 files. localStorage: Wrap in try-catch . Uncaught throws crash the whole page. CDN scripts: Always <script defer> . Without it, slow CDN blocks rendering. Gumroad publish: curl returns false every time. Use PowerShell. Buy button: Popup connects the page to Gumroad. Redirect breaks postMessage . Packed 5 templates with everything pre-configured: Browser Tools Starter Kit ($19)