After using the American Express Network's Auto Purchasing Program, which is administered by www.zag.com, it didn't take long to discover that the participating dealers which provide quotes with “Price Protection Certificates” for your configured vehicle have no intention of honoring those prices. They merely use the service as a source of sales leads.
FYI, other buying services that go through Zag.com include USAA, Consumer Reports, Overstock, Bank of America, Sallie Mae, NEA, and others.
Dealers not honoring their price certificate allegedly violates their contract with Zag, but when I called Customer Service to inform them of this nonsense, they told me there's no guarantee if it's on a non-inventoried vehicle, as if it's my fault. Unfortunately, there is no such language in the disclosure statement. Not that it matters, either. Because I then changed my vehicle to one that was in a dealer's inventory, and they still declined to honor their quote.
So, Zag's business model is bogus. Which is really of no surprise. The only use the service has is to guide you in your negotiations with dealers. I took my price certificate to numerous dealers until I found one to beat the lowest quote, making them think that so and so dealer was honoring their quote to begin with. All's fair in war, love and car buying negotiations.