Interesting. I was under the impression that Costco's buying service worked like cars.com; you simply tell them what you want, generate a certificate, and then take it to the nearest participating dealer to get your car.
No. It has nothing to do with cars.com. Costco has specific dealers, and within those dealers has specific people that Costco trains and authorizes to handle Costco customers. The Costco guy I spoke about the experience with was not at all happy that my dealership let someone random deal with me, someone not on their list, and that it ended up in the back-and-forth thing with "my manager".
The process is simple: give them what you're looking for, and they'll pass you along to their local dealer if they have one. (If not, you're screwed.) The dealer will call you to set up a time; you go in and show your Costco card. At that point the sales rep--who is supposed to be authorized by Costco to do this process, remember--shows you the Costco pricing, right there. No back and forth.
I had already drawn up a spreadsheet for all this and had used it at other dealers before I hit the Costco dealer. And like I said, the Costco site showed all incentives, right there. So when she gave me the "it's invoice less $900" price (that she had to go back to her manager and get--whoops!) and said "and we have some incentives on top of that" and came up with a "final" price, I was ready. I knew about the $3500 dealer cash. I plugged it all in and realized she wasn't giving me all the $3500, and she was $1100 higher than my first offer from my first dealer. I told her so. She asked for it in writing--well, what do you know, I happen to have that.
She takes that back to her manager and comes back with, "whoops, um, yeah, well, we were looking at the wrong paperwork" and magically she matches that first offer. That whole BS process is specifically NOT supposed to be part of the Costco process.
Costco will also give you 50% off on accessories, up to $400 off (I think), should you complete their survey. I leveraged that with my selling dealer (who is NOT a Costco dealer) to get the Homelink mirror--installed--for half off.
Costco also gives you 15% off service, accessories, and parts anytime at any Costco dealer or service organization.
I'm surprised that their site actually lists the dealer cash as such, since all the manufacturers treat dealer cash programs as privileged information. The dealers (even the ones who'll give you the whole incentive without negotiating) don't refer to it as a MFR to dealer incentive in their advertising, but they will make it look like they're giving you a discount out of the goodness of their hearts.
The place I bought it from, who gave me a great price right out of the gate (they were the first I went to, and everyone else--even the Costco dealer--was higher), never said "dealer cash". They just gave me a great price right up front.