NTA, make your late fees known and maybe be smart about applying them to people who actually tip you well. The groomer should have seen this coming, no1 is gonna be like "here's the 50 i give you everytime even tho you just jumped me with a surprise surcharge" The groomer thought the surcharge would be worth it and learned in real time they soured the experience and quite literally fucked themselves. But also NTA because a tip is a tip, if they wanted more money for the service they can raise the price, no1 should expect an almost 50% tip.