This is an improved deal (exp. 10/4) at Costco vs the previous one (exp. 9/27), because it provides a bill credit for the activation fee.
In general, it is almost the same Sprint deal as the Best Buy one, except this one has a waived activation fee and the "TYLT Power Essentials Pack", but the Best Buy deal has a good forum thread on Slickdeals to reasearch.
Best Buy Sprint Iphone XS forum thread: https://slickdeals.net/f/12051147-bestbuy-sprint-iphone-xs-max-4-20-mo-or-64gb-iphone-xs-various-colors-24-mo-installment-plan-net-0-phone-after-24-months-credits