Yes, I can the scariness but charge them for it upfront - folk would rather be warm and not have to scramble for two pound coins or credit cards....thinl you need to do some mkt research there - perhaps starting with a question on here, ratter.
I'll give you an example - remember faxes? (Not quite the same thing, I know - However). Hotels that charged me for incoming faxes, which, ultimately were out of my control, then I would give them a warning to remove those charges or never come back. It really niggled me as a point of principle - I would have preferred an average of let's say £5 on my bill and have the convenience.