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

1. User hits free limit → PRO modal
2. Buy button → Gumroad popup (window.open, NOT target=_blank)
3. Payment → Gumroad postMessage with license key
4. message listener catches key
5. Key verified via Gumroad API (/v2/licenses/verify)
6. 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.

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Packed 5 templates with everything pre-configured: Browser Tools Starter Kit ($19)