I think the rule is that, when a business charges a consumer a penalty fee of any kind, the fee must relate to actual costs that the customer's actions imposed on the business. A business could estimate the cost of bananas, but they couldn't use the sale price if that was usually ten times the cost. This applies to things like bank overdraft fees and hotel cleaning charges as well. The idea is that consumer contracts can recover costs, but aren't allowed to punish people.

I don't have a source for that.