How I add Polar (Merchant of Record) + Stripe to any Next.js app in 10 minutes — without an SDK

If you're a solo dev outside the US, "just add Stripe" is rarely just. Stripe doesn't onboard sellers directly in a lot of countries, so you reach for a Merchant of Record (MoR) like Polar or Lemon Squeezy — and then you hit the second wall: the official SDK throws fetch failed the moment you deploy to a serverless platform like Vercel.

I shipped paid checkout on two live products this way and got tired of re-solving the same problems. Here's the pattern that actually works in production.

1. Skip the SDK. Use native fetch.

Most payment SDKs assume a long-lived Node server. On serverless, the bundled HTTP client and keep-alive sockets misbehave and you get opaque fetch failed errors. The fix is boring and bulletproof: call the REST API directly.

// lib/polar.ts