It definely has to do with the speed rating. Go somewhere else. I don't drive along at 130 mph for long (or even short) periods of time so I replaced the factory tires with Michelins (same type) but with a lower speed rating (118 mph).

You think Costco would just have you sign a waiver..... (*&$#* damned lawyers)
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1998 S90 062,700 1995 964 142,100

Personal experience: They go by what the computer shows the car is "supposed" to take, or by the speed rating on the sticker on the passenger door if you press it. They said I had to use the "H" (130MPH) speed rated tire unless the tire sticker indicates a lower speed rating is allowed. The door sticker shows a T (118MPH) speed rating for my car. Armed with this info they should allow a tire for the speed rating marked. (I don't know if the 93's were marked with the speed rating requirements.)
After two sets of COSTCO tires, two balance and rotates, and a warrantee replacement, I have nothing but positive things to say about their tire service.
They use hand torque wrenches only, and the manager double checks all torques. If your car requires 65 ft-lbs, make sure they note that on the order.
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Tom F

Costco gave me the same story when i went in for tires too. They said I need V (150PMH) rated tires and would not sell me the H rated (128MPH) tires that they had in stock. Also they would not let me have different type of tires on the front and the back with I thought was strange (I only wanted to replace the rears). I'm never going to drive nearly that fast so i don't see the point. Go to tirerack.com and have then installed at a local shop. It turned out it was cheaper than even the H rated tires Costco had with shipping and everything.
Good luck

When I went to buy tires for my 95 964, the guy at Costco told me that according to his listing, that I could only use tires with the "Z" speed rating. The tires on the car at the time did not carry that rating. They are nice tires but have a very low wear number and are in fact wearing out rather quickly. Next time I need tires I am going to see if I can get less expensive and longer wearing tires. I do not need tires rated for 100+ mph.

I believe this to be due to Costco losing a lawsuit, details of which are now fuzzy in my mind. I think they give employees no latitude in substituting tires on any kind of car. I doubt seriously that they are trying to engage in bait and switch or upsell.

Check Tirerack.com I can't imagine it is a load issue. I'll bet it has something to do with his revenue targets for the month.

It's called "ADM", or additional dealer markup. Get 'em in at a lowball price and then switch at the last minute ("oops, sorry sir, we can't get that model in orange.")