So your cat is a better tech than the guy at the repair shop charging $60 an hour. Tell her I said good job and maybe she can look at my printer next.

That line about the whole thing coming down to he said, she said is the part that really gets me. It is not even about the $400 by itself, it is that you had zero ground to stand on once the number changed on the spot. And it gets worse the bigger the job is, because a fence post swap is one thing but a full tile rip out means dump fees, maybe a permit, maybe a guy finds rot behind the wall once it is open. All of that stuff is exactly where a verbal quote falls apart, since nobody remembers the same conversation. What I would add is that the written bid should still spell out how changes get handled, like who signs off on extra work and what happens if they hit something unexpected. Otherwise you get a paper with a price on it and the same argument the second a surprise pops up. Getting it in writing is the start, but getting the rules for changes in writing is what actually keeps you out of that mess.