We have bought our last four cars through the Costco Auto Program. I must say it is a very slick program.
#1 Know exactly what car you want
and what you want in the car and
what you don't want.
Email or check the Costco website to see what dealer they want you to go to and to what sales manager they want you to contact. This is very specific. You can't just go to anyone anywhere.
#3 Go to the dealer and see what they have on the site in the car that you want.
The Dealer gets "the book" and your price is precisely $500 over what is on the book.
#4 Your price is what that number is minus what they will give you on a trade in.
No muss, no fuss, no haggle. Take it or leave it.