Okay, I can already tell you I'm going to piss someone off with my comment, but here goes...
What annoys me is that so many people are so freakin' eager to take offense to someone wishing them well. And that's all that girl was probably doing. Is she only allowed to say "Have a blessed day" to fellow Christians? If you're Jewish and say "Shalom", are you only allowed to do that to fellow Jews? Is it okay for me to take offense because I'm NOT Jewish? (I can already see it: How dare you say "Shalom" to me! Can't you tell with my last name of Smith that I'm not Jewish? You'd better say "Have a nice day" next time I come visit your store or I'm taking my business elsewhere!) This is about as ridiculous as ME demanding that YOU tell me "Have a blessed day" because I am a Christian.
Gimme a break people!
When you think about it, every action you take is a choice. You are basically CHOOSING to be offended over it. There are so many other things to expend your energy on rather than CHOOSE to be offended by something so little. How hard would it be to say, "Thanks, I am having a great day!" I can guarantee she'll look at you, smile, and be happy for you.