You didn't include any technical details in your post.
Tire sizes?, which wheels you are using?, etc..
Costco is not really a tire shop. For example they will not mount nor repair tires they do not sell.
In your case, just take your tires to another tire shop to get them mounted.
I just had a heck of a time purchasing larger than stock BF Goodrich tires at Costco in Calgary.
I went slightly larger than stock with a slightly larger than stock wheel & wanted 1 tire mounted on 1 wheel which I wanted to bring home to test fit in my own garage.
At 1st they had no clue why anyone would want to go larger than stock & they had never heard of anyone ever doing a test fit.
The discussion about wheel offset & backspacing was frustrating also.
The young guys in the Costco tire shop are very narrowly trained. They just follow process & go by their what their on-line system tells them.
For example if you wanted to plus or minus size tires & wheels they wouldn't have a clue what your talking about. Nor would they know about other non-stock or performance applications.
Nor did they know the difference btwn static & dynamic balancing.
Costco tire prices are very good for Michelin & BF Goodrich tires but their shop is all about high volume, quick in & out layman folks.
Not for performance minded folks. I spent some frustrating time there to save a few bucks...
...Joe
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2006 Premium White TSX, 6MT. Wifey's daily driver.
CT short shifter & aluminum shift tower bushings. M3 lip, debadged.
DIY CAI/K&N fillter. DIY exhaust (2nd resonator removed). Progress RSB.
Summer: 17x7.5, ET50mm Sport Edition's. 225/50ZR17 BFG Super Sport A/S.
Winter: 16x6.5, ET45mm OEM RSX. 195/65R16 Nokian winter radials.
Red "not an ashtray" painted black.

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