it doesn't store the purchase, it stores the email, then parses it into a list of purchases. Also if you delete the email, the purchase list remains.

I don't need Google's help to search through my purchase emails. My emails are well-filtered into categories. And where that's not sufficient, search terms aren't difficult to use.

Google automatically collecting a list of purchases goes beyond what I expect an email provider to do.

Personally, I'm not a fan of email being plaintext and think everyone should use encrypted email (GPG, ProtonMail, Tutanota, etc). I think it's absolutely inappropriate for Google to track such things even if it has access, and it shouldn't have access IMO in the first place.