"custom" can kiss my ass
If someone has a shit poor attitude then I'll pay the price for the food and make them dance to earn 5% or they can eat cat food.
Again, call it a surcharge, call it a mandatory wait staff customer payout, but it's NOT a "tip" and it's NOT a "gratuity". Simple. Change the damn name of it and stop pretending it's correct to have a thing called a "mandatory tip", because it isn't.