I've had other services done that were Costco replated- a complete HVAC system. Plus I bought 2 cars with their auto buying service! All were smooth purchases. The HVAC esimate was competitive (maybe a couple hundred less), the 3rd party company had rave reviews on Angie's List - and I even got an $800 costco gift card!!
A suggestion: if you purchaae the carpeting, up your membership to executive. You'll get a nice rebate check at the end of the year! When I bought my HVAC system I changed my membership to executive for just the month it was installed!!!! It cost a couple dollars to make the change and I got around $100-125 back.