Hey,
I have personally run my own Shopify store and would be happy to help with the payment testing.
To make sure I don't purchase the $20 product and get scammed. I propose an alternative where you accept my bid on the project and you pay me an upfront amount of $15. Then once I buy the item from your store, for an amount of $20, you finish the project by paying me the final $15.
Hopefully, you are OK with this proposal, and I can purchase this item from your store.
Kind Regards,
Max