The service that sent that receipt was one of those 3rd party services for small vendors. You go to a bakery and they are able to magically email you a receipt, that sort of thing.

So I very much doubt the person put their email in incorrectly, but I understand your point.

The exact mechanics of how it happened I don't know, but I very much disagree with firstname.lastname@gmail.com and firstnamelastname@gmail.com resolving to the same address.