I think she did the classic script "fight-picking" with you, over something very minor.
You need to be firm (not "tough"), but on the BIG things, not $100 of a bill. Learn to operate not from a standpoint of "How will she react if I do/say this? Will she be angry? How will her reaction make ME feel?" and instead from one of "What is The Right Thing to Do in this situation? What is the thing that God Himself would have me do if He were standing right here in front of me?"
