I used to be an avid believer in Costco Tire Center but not anymore.Costco is best for tires. Free road hazzard. Free repairs. Free nitrogen. Their prices are competitive with tirerack, and mount and balance is $14 per even for runflats. As Ard said, they did you a favor.
I'd say this kind of stuff is hit or miss with many chain stores.I used to be an avid believer in Costco Tire Center but not anymore.
They refuse to change the sizing of my 335i tires from the OEM. Not Tim size just width of the tire +1.
They also won't mount go-flats without me having a spare. A pump/goo kit won't work for them.
Use America's tire co. They do most of what Costco does and is a lot more willing to work with you.