Costco's tire service is to help them selling more tires which is one of the thousands items they sell. They have very average equipments and those tire "techs" have minimum experience on few tires due to very limited brands (4) they carry. A tire tech works for a dedicated tire shop such as Discount Tire Store knows much more on tires!
One "tire tech" I spoke to at Discount Tire thought that "T" was a higher speed rating than "H", presumably because its later in the alphabet. When I corrected him, he tried to tell me that if I'm not driving over 100MPH, the higher speed rating makes no difference.
I still use Discount Tire often, as they are convenient, easy to work with, and seem to have a lot of latitude on pricing.