I always breathe a little sigh of relief when one of my customers says they do have an email address to receive the receipt. However, many more these days, are not utilizing email. Therefore, the option to enter a phone number instead of email in the receipt process would be fabulous because the following is what I have to do to send them a text (or Whatsapp) message.
1) Finish the receipt process then
2) go to receipts and open the receipt then
3) take a screen shot (sometimes 2 if the receipt is too long) then
4) open text messaging (or Whatsapp) then
5) ask for their phone number then
6) enter it and attach the screen shot(s)
This is just enough extra steps/time to be somewhat of an annoyance for the customer and any other customers who are waiting in line behind them.