Hi Jack,
Since the price field in in the products collection with Wix Stores is read only, I can suggest building a product page from scratch instead of using the Wix Stores default product page to build this. When you set up, you can you use the Pay API to allow customers to check out. Take a look at the tutorial here to see how the Pay API works.
This is just one way you can go about accomplishing this but there should be other methods you can try. If you feel like cannot make it alone or need some help building this, please consider hiring a Wix Partner to help you build your site.
Best regards,
Edward | Corvid Team