This is kind of the tradeoff you need to make when launching a product though. You cleave off some of the product's margin & send it to a third party so that you can get the thing launched. If it's unsuccessful, that's fine, you'll pay no money to the vendor. If it's successful..? Great! Now you can afford to pay someone to build a checkout that doesn't cost me thousands a month in fees.

Stripe takes 1.5-2.5%, so if you're sending them 1,000s a month, your revenues from that checkout are approaching the $millions p/a. Certainly enough to hire an expert in the domain.

This means you’ve done everything absolutely fucking right