What is the problem? If the customer has a valid receipt, the customer is entitled to have the repair / refund. There must be no difference to buying something in a physical store.
What consists of a "valid receipt" for a digital purchase? An email? A printed copy of an email? Either can be trivially forged if the company has no way of verifying the information.
Usually, when I order something online, I get an receipt via mail and paper (with the shipment). Mail and paper receipt contain order details which can be used to prove the legitimacy. And you can always sign the receipt (plain/text or attachment with the mail, QR code with the paper receipt).
It is important not to delete the order data, but to anonymize the user data. Your shop software should be able to handle this. Otherwise, I would use another product ;)
