You should let me send the first card for free, but make sure I make an account and then follow up with it. Check out Touchnote, which is an iOS app that lets you send people cards using photos you take with your phone. They have a good startup experience that converts you into a return user pretty effectively.

It might be interesting to put a note in the card that tells mom that she can send a card back to me by emailing a special address as well. It would be nice if I could pick something for her that she feels is familiar, for instance myname@sendwrite.com, as opposed to the anonymous mail@sendwrite.com, which feels like I'm emailing a company.