Do you want me to come back later?

I was out of town at a conference and needed to sign and scan a document. I went to the conference's concierge desk and ask them if they could scan and email the document for me. They said yes, but then were taking a while to do it while trying to juggle several tasks. I couldn't keep waiting, but since they were doing me a favor I didn't know how best to tell them to hurry up. As the expression says "Don't look a gift horse in the mouth". But then it hit me. I ask the person helping me if she wanted me to come back some other time to deal with this. She realized that she had been keeping me waiting and said no, she would finish it right now, which she did. It is a pretty universal rule that the nicer you are at getting your points across the likelier people are to help you...