what should I use for auth and billing in a SaaS startup

For a SaaS startup, the usual best approach is:

Auth

Use a managed auth provider unless you have a strong reason not to.

Good options:

• Clerk — fastest to ship, great dev experience, good for B2B SaaS
• Auth0 — very flexible, enterprise-friendly, can get expensive
• Firebase Auth — simple, solid, especially if you’re already on Firebase
• Supabase Auth — good if you’re already using Supabase/Postgres
• Cognito — powerful, but usually the most annoying to work with

My default recommendation

• Clerk if you want speed and a polished frontend auth flow
• Supabase Auth if your app is already Postgres/Supabase-centered
• Auth0 if you need enterprise SSO/custom auth complexity

Billing

Use Stripe for almost every SaaS startup.

Why:

• subscriptions, metered billing, invoices, coupons, tax handling
• works well for cards and global payments
• huge ecosystem and docs
• easy to integrate with webhooks and customer portals

Add-ons you may need

• Stripe Tax if you need sales tax/VAT/GST handling
• Lemon Squeezy or Paddle if you want them to handle more of the merchant-of-record/tax burden
• Chargebee only if you’re doing more complex billing ops and can justify the cost

Best “startup default” stack

If you want the simplest solid setup:

• Auth: Clerk
• Billing: Stripe
• Database: Postgres
• App: Next.js / Remix / similar

Choose based on your situation

Use Clerk + Stripe if:

• you want to ship quickly
• you want good UI components out of the box
• you’re building a standard B2B SaaS

Use Supabase Auth + Stripe if:

• you want to keep things integrated with Postgres
• you’re comfortable building more of the auth UX yourself

Use Auth0 + Stripe if:

• you need SSO, enterprise features, or advanced identity controls
• you’re okay paying more for enterprise-grade flexibility

What I would avoid early

• building your own auth
• building your own subscription/billing system
• using multiple overlapping providers unless you truly need it

If you want, I can also suggest the best auth/billing combo based on your stack (Next.js, Rails, Django, mobile, etc.) or based on whether you’re B2B, B2C, or enterprise.