Wow that's a lot of selling for Costco. Do you work there? That's really odd. Also let the credit card thing go. People conscious about their finances will not be willing to put inquiries on their credit by opening up revolving charge accounts just to get an "extended warranty" for their new Mac. That's why the nation/world is in such high debt. And while you were posting the benefits, you neglected to post the T&C about the warranty. I looked it up and just as I suspected the customer has to pay Apple upfront for the repairs and they may not have the money to do so. Recommending this is bad advice.
Anyways what you failed to mention is the WORST part about buying from Costco when it comes to the new iMac. There's only one color (blue) and you no BTO options so it's not ideal for many people.
Maybe he does, but I don't work there. Although I freely admit I'm a stock holder just like I am for a lot of the places and companies I like. I liked them (PriceClub!) long before I was a stock holder, not because I'm a stock holder.
People who understand finances know how to use the system to their advantage. Did you know there is such a thing as paying off your CC off every month??

I haven't paid a dime in interest in 30+ years and have been able to take advantage of cash back, airline miles (although no longer have any cards that do that), extended warranties, additional discounts from merchants, fraud protection, free rental car insurance, free credit reports, free annual spending reports etc from various cards I have had over the years.
Also yes you have to pay upfront for repairs, however you have to put that on the same card anyway so you get a grace period and even if it takes longer to do the credit back most cards also credit back the interest on that portion.
Also yes, Costco is limited to standard configs. Which is why many of us complain when Apple under configures those configurations or makes some things like VESA mount a BTO option only. However you are incorrect about the color options. They do offer some other color options, however only the Blue is considered in stock currently. I'd prefer they just list them as out of stock, but instead they hide the other options. I know they were offering green and silver at least as I was trying to decide on those 3 for my parents.
Also BTW - if you did want AppleCare + you can get that too and usually at a discount (for example right now it is $134.99 vs 169 through Apple for the iMacs).