I depends. For projects without much complexity and a single platform I usually aim for 2-3 weeks but end up around 1 month :).

Awesome idea 💯💥✨

Nice 👍👍! One thing that would be nice is a live demo of the code used to achieve the thumb (and maybe able to edit parameters).

I've only used Stripe and only for consumer payments not businesses. I export all payment data from Stripe which gives me the country of the card (whatever that means) and have a script that calculates vat to pay per country :)