I think I've hit upon the crux of the problem -- my problem -- with retail; saving people from themselves.
Me: What is the amp rating of the fuse you need to replace?
Them: Hm. I think ...I think it's 4 amps. I lost the original.
See, if I point out that putting a fuse rated larger than the original will result in the energy not being clipped out of the circuit -- the exact reason why you have a fuse -- in case of an overload, I'll get one of two things:
(a) A tortured "I don't want to go alllllllll the way back home or;
(b) A blank stare which indicates that the customer believes I can somehow guess the correct value and will blame me when the device is destroyed.
So I just give them a hindu cow stare and let them pick the wrong fuse.