12 weeks is now considered the norm, it allows the kitten to be better socialized with it's litter mates and Mom before leaving home.
12 weeks is absolutely the right age. Many shelters will let them go at 8 weeks because they are capable of being separated from mom/littermates (or they were orphans to begin with), it saves them money and people like the idea of little tiny kittens...making adoptions much easier.
But...an 8 week old is likely to not be well socialized. Think about it...they spend their first 6-7 weeks learning survival skills...getting eyesight and hearing, learning to walk, getting teeth, learning to use a litterbox, getting weaned onto food, learning to identify danger etc. There's not much time there to learn social skills. Weeks 6-12 is the time they learn things like not to bite too hard, how to be brave without mama right there, mama teaches acceptable and not acceptable behaviors etc.
BTW...a breeder that sends kittens home before 12 weeks is not in compliance with CFA or TICA and it's a pretty good sign of a backyard breeder.
It is nice to see a person who is buying a cat and has researched the breeder. I hope you post pictures when you are able to bring your baby home.
I would expect microchipping and neutering as well, otherwise all normal.The kitten will have 2 set of shots by then, dewormed 4 times and health clearences.