We’ve noticed a recent trend of retailers asking customers at the point of sale whether they would like to receive a copy of their receipt by email. We would advise you not to do this if you value your privacy. Even though they’re not really supposed to, they’ll probably add you to their marketing database. They’ll spam you. They’ll share it with their partners. It’s their responsibility to put a paper receipt into your hand as proof of purchase.
If you do want to get a copy of your proof of purchase sent to you via email, don’t give them your primary email address. Give them a throwaway address.