We chose to use Partial.ly for payment processing. First, we tried to use Litch. But with Litch we were not able to transfer the user from the payment page all the way to the GCP function and the Firestore database.
Partial.ly combined with Stripe works quite well. The only problem is that they seem to have made it a bit too complicated to set up subscription offers.
Why didn't we just use Stripe for subscriptions? We wanted to get something up fast so we opted to use something that could process subscriptions without developing our own node.js server that receives webhooks from Stripe. Why reinvent the wheel? So, therefore, we selected Partial.ly for payment processing.
This is only the beginning. The next phase, marketing is also a tuff nut to crack. :-)