If you're a developer, I don't recommend using
extensionpay.com due to the fees issues. Stripe already takes 5% from each subscription user, and extensionpay takes an additional 5%, which is unacceptable for me.
Although I had no prior experience with firebase and stripe, they are actually straightforward. I spent only 2 days using firebase and stripe to build a generic subscription system, which reduced the subscription fee by 5% for each user. Since I started monetizing, this 5% fee has already saved me $200, lol.