300Mb/s AT&T Internet from (at) Costco

There are 3rd party non-AT&T / non-Costco resellers at my local S.F. Bay Area Costco hawking AT&T 300Mb/s for $55/mo.

They claimed that if I sign up through them, that the $55/mo will NEVER go up (Costco Guarantees it the fixed price). AND, whenever I need to contact customer service, I will deal with Costco Customer Service, not AT&T (implying this is better than dealing with AT&T directly).

Has anyone signed up for Internet Only through a Costco seller? It this correct? I would be surprised if Costco would let a scammy company on site, but "Trust and Verify".

Anyone had experience with this?