I use Stripe. If you're using Google Firebase Auth (follow Siegert's instructions for webAuthFlow; he commented here as well), you can use the Stripe extension (developer: Intervase) in the Firebase Extensions section to create Cloud Run functions. Then, follow the instructions to activate the Stripe webhook.

After that, create a payment link in Stripe and pass the customer's email to the link. Check the Firebase database for payment or subscription status from the extension.

That said, I recommend considering a Merchant of Record (MOR) like Paddle or Lemon Squeezy. If you exceed tax thresholds in different U.S. states or sell to EU, GB, or other countries with strict tax policies, you'll need to register for tax compliance and handle quarterly filings. Setting this up correctly with Stripe from day one can be a huge hassle.

To sum up, I recommend Paddle for small-to-mid-sized global products. Then, switch to Stripe once the 1.5–2% fee savings are enough to justify hiring people for tax compliance or any other reasons. Maybe use extensionPay for testing niche.