I tried to do this in the beginning of July. The first thing is to go to Costcoauto.com and find a participating dealership in your area. Then follow the instructions and eventually a dealership will contact you. Make sure to check the invoice price on costcoauto's website.
And for whatever reason, the invoice price that costcoauto.com, edmunds.com, kbb.com have for the GTI are about $500 LESS than what the dealer showed me :iono: . On top of that, the salesman said that since the GTI is such a hot car, they are selling it for $750 OVER their invoice price (he showed me a pre-printed form with all of the invoice prices and the costco prices for all of the VW, some were only $100 over dealer invoice).
So for a DSG, 0 package, with monster mats, the quoted invoice price was roughly $22,600. So add $750 to that ($23,350) and that was the quoted price. IMO, you can do a lot better by going to edmunds.com and get dealer quotes online through that website. That's what I did and got a DSG, 0 package with Sirius for $22,970. A much better deal than I got trying to go through the costco program. :headbang:
Let me know if you have any other questions, as I'll be glad to share more of my buying experience.