Dear Husband on Table 52,
Did you know, the fact that you had a $50 gift card and only paid $30 out of pocket does NOT mean you tip off the $30? Guess what. I did $80 worth of service, including surprising your wife with a piece of chocolate cake with "Happy Birthday" written in chocolate which will probably get you laid tonight.
I hope your wife didn't see you leave $7 when I brought you back $9 in change.