Support simple purchasing parity built-in

[sindresorhus]

I'm currently using Gumroad, but I would like to eventually move to something else. One feature Gumroad has that my customers appreciate is Purchasing Power Parity. I saw that Polar recommends using an external service, but that's a bit overkill for my simple needs, and also risky (another service that may go away) and too expensive for what it offers. All I need is to just enable the purchasing parity and be able to set a maximum percentage.