It doesn't really matter what time you serve snack or if mom is pissy about it. The fact is that is your rule and she KNEW about it so it is up to HER to ensure that her kid is fed either by arriving before your snack time or ensuring the kid is fed at home before she arrives. It's really not that hard to chose one and do it.
I would NOT be charging her more per day and then making her princess a special snack. You give options, it's HER choice to not take advantage of them.
I would have done the same thing.