Costco's phone center is run by a third-party and phones typically are sold through the 4 (3) major carriers. As such, the deals they offer tend to be a variation off of those the carrier is offering, typically with an extra Costco "incentive" added.

In particularly, while the carrier typically offers deals where they provide the benefit over 24 months, buying at Costco they frequently provide that benefit "up front" (as a rebate, so it takes a couple of months to process) if you are buying the phone outright, rather than "paying" it out as bill credits over 24 months.