I love it when custys clean up after themselves. It almost never happened to me, mostly because we really weren't mean to let customers clean anything so even if they did say, "Oh let me do that."
I had to tell them it was OK and of course they rarely argued.
There was one time a little girl dropped some juice on the floor in front of my demo counter and the mom argued with me saying she'd clean it up. I gave her a cloth to clean her daughter up but said I'd do the floor.
Bear in mind that I was hugely pregnant at the time but short enough to mostly hide that fact behind the counter.
She did begrudgingly agree to let me clean the floor until I came round and she saw my belly at which time she said, "Oh no you don't! Get back there and sit down while I clean this up, I'm not letting you get down on the floor to clean my mess in your condition!"