There are no format requirements concerning how the customer gives the e-mail address. The customer can provide the e-mail address or other similar information verbally. It is also not a requirement that the customer must be identified via the payment method. The important consideration is that the receipt is always presented to the customer on paper or sent electronically; see Section 2-6 fourth paragraph of the Cash Register System Regulation:
It must not be possible to conclude a sale without the system printing a sales receipt.
If an electronic receipt is chosen, it is not sufficient for this to be placed in a Recycle bin. The following is stated in the remarks concerning Section 2-6 fourth paragraph of the Cash Register System Regulation:
The cash register system must be able to print the receipt onto paper, but there may also be an option for an electronic print-out. An electronic print-out is made when the receipt is sent from the cash register system in such a way that the customer can receive the receipt in electronic form, e.g. on a smartphone or via Digipost.