I don't see what's wrong with saying "no problem" honestly. In statements like this, so much more is conveyed with your tone of voice, facial expression and body language. If you meant it, you could have said "have a nice day!" "sure thing!" or "you got it!" or even "well, see you around!" and it wouldn't matter... ultimately what you're really doing is acknowledging the thank-you, and not leaving the person hanging.