For eBay orders, when printing a label in the till during dispatch, the email address field is auto-populated with the store's own email address. This is not the case for Amazon orders, so the cashier has to enter this manually each time. If you would like to see this changed, please vote and comment.
(Note: The customer's 'real' email address is not used for either eBay or Amazon orders as these are not available from those platforms due to their data protection rules.)