The locations are staffed by actual corporate T-Mobile employees just like regular stores, with no third party companies involved.
update: Costco Locations Selling T-Mobile 1/13/23 - Google My Maps
无激活费:节省 $35刀
Costco shop card: $150 new activation/$75 upgrade ( purchase a new device on EIP (new activation or upgrade) at participating Costco/T-Mobile exclusive pilot locations and receive up to $150 on a Costco Shop Card”.)