You can send as many copies of an e-receipt as is required from the customer's purchase history, which you can see on their customer record in the Back Office.
Resending an e-receipt
1) In the main menu in the Back Office, expand the Customers heading, then click on Customers.

2) Find the correct customer using the wildcard search or the filters in the column headers, and double-click that entry to open their customer card.

3) Click on the Purchase History tab at the top.

4) Find the correct transaction and click on Actions, then on Re-send e-receipt.

NOTE: The customer account must have an email address saved in order to send an e-receipt.