Similar problem here.
My Windy.com account was created while ago via "Sign in with Apple" so I have some xyz@privaterelay.appleid.com email address in my account.
In Safari I logged into my account ("Sign in with Apple") and purchased the Premium subscription. As my email address I entered this xyz@privaterelay.appleid.com address and payed via PayPal (which is a different email address!).
I got a confirmation email from PayPal for my purchase but never received an email from Fastpring with some code I could use to re-enable/restore my premium subscription.
I checked all spam folders, too – no email from Fastspring.

Now what? 😳