It certainly wouldn't be natural for a shop assistant to say it in the UK either. It would be natural for someone in a casual situation, who sees someone else struggling with something, and who is offering to help. (That's not the same kind of help/assistance that's on offer in a shop.)
John (struggling to hang wallpaper): Aaarrrgh! This is driving me mad. It's impossible to do it on your own.
Paul (sighing and trying not to laugh): Oh. Are you having trouble with it. Shall I help you?
John: No. You just carry on sitting there, reading the paper and letting me do all the work.
Paul: Really?
John: Of course not! Get up and help me, you idiot!