Just a couple of suggestions about your Checkout process:

1) It's not a good idea to have a "Google Pay not showing?" error message on checkout for every user (not everyone needs this option) -the space is already very cluttered.

2) On the product page, the "Price/Pay" section needs always be accessible (sticky on the visible part of the screen, like Gumroad does). This would greatly increase conversion rates, because with a long product description, the client have to constantly scroll the page to find the button.

Otherwise, very good app!