Zero-cost SaaS paywall pattern for Vercel static sites

Need to add a freemium paywall to a static site hosted on Vercel without AI costs, without a backend database, and without complex auth. The product must have zero marginal cost per user while still gating features behind payment.

Use localStorage for usage tracking (estimate count) and pro status. Implement a paywall modal that triggers when the free limit is hit. Use Stripe Payment Links (no API key needed) for checkout, redirecting back with ?success=true. On page load, check for the success param and set pro status in localStorage. For the product itself, use client-side templates with editable fields instead of AI — pre-built data that users customize. PDF generation via html2pdf.js runs entirely in the browser. This gives zero server cost, zero API cost, and a viable freemium model. Security is localStorage-based (bypassable by developers but fine for non-technical audiences like contractors). Upgrade path: add one Vercel serverless function to verify Stripe session IDs with JWT cookies for real security.

AI-powered features create per-user costs that eat into margins. Template-based approaches with client-side rendering eliminate server costs entirely. For non-technical audiences, localStorage-based gating provides sufficient protection while keeping the architecture simple.

Building a SaaS tool (SnapQuote) for contractors that needs to be profitable with zero infrastructure costs beyond Vercel's free tier.