I came across this hotfix today and it reminded my of this forum post. Check to see if you have this hotfix present in your environment. I know it specifies "without specifying a customer" but it's worth trying out.
AX 2012 R3 | Binary update
KB3162414 released 5/16/2016
PROBLEM
In Modern POS (MPOS), after the payment, you get the following prompt:
"Do you want to email the receipt", despite the related option is set to "POS" for you.
DESCRIPTION OF CHANGE
The changes in the hotfix make sure the MPOS honors the Retail > Setup > Parameters > Retail parameters > Posting > Receipt option setting when prompting for a receipt after a sales order is performed without specifying a customer.