Tipping seamstress - please help

I am going to pick up my dress tonight and I was wondering if people usually tip the seamstress and if so, about how much. This is actually my cousin's MIL (I didn't really know her before this- but not through a shop) and she said it'll be $80. Do you think that if I gave her $100 that's good? Or maybe $125?
Thanks!

Thanks so much!! I am just about to leave to drop of my final payment!! and was hoping someone could help me before I left.