The best way to locate deleted items from your inbox, whether they be receipts or regular emails, is to check three folders:
- All mail
Depending on what email client you're using, deleted mail will first go into one of these folders before being eliminated entirely.
If you can't find you receipt in any of these folders, there is still hope.
A bank statement showing your transaction can also legally be used as proof of purchase. If you paid by card or bank transfer, this should show up.
Otherwise, if you're having no luck retrieving your receipt, you could always ask the store themselves. They are required to keep a merchant copy of the receipt, and should be willing to provide you with a copy if you are persistent.