How do you build subscription billing in a MERN SaaS?

We integrate Stripe directly into the Node.js backend: plans, trials, proration, coupons, invoicing, dunning (automatic failed payment recovery), webhooks with idempotency keys, and Stripe Tax for regional compliance. Every Stripe webhook is handled idempotently, so duplicate events from Stripe's retry mechanism never create double charges or double provisioning. For international SaaS products that want a merchant-of-record model (Paddle handles VAT and sales tax automatically), we integrate Paddle as an alternative to Stripe. Billing is built in the same sprint as tenant onboarding, not added after launch.