I couldn't find one for Paddle to use but I'm sure I saw a few for Stripe. I wanted to use Paddle because they do EU VAT for you. I could be tempted to open source it though.
The gist of it is when a subscription is purchased a web hook triggers a Firebase Cloud Function that stores the subscription data (i.e. email address + subscription expiry date) in Firestore then when someone logs in via Firebase authentication you check if the email address is linked to a valid subscription. When the next payment is made to the subscription, you bump up the subscription expiry date. I've found there's little that can go wrong once it's up and running.
The gist of it is when a subscription is purchased a web hook triggers a Firebase Cloud Function that stores the subscription data (i.e. email address + subscription expiry date) in Firestore then when someone logs in via Firebase authentication you check if the email address is linked to a valid subscription. When the next payment is made to the subscription, you bump up the subscription expiry date. I've found there's little that can go wrong once it's up and running.