This deal has been out for a while now but I wanted to shed some light on it. If you port your line from a different carrier to AT&T, trade an iPhone 7 (any amount of storage) or a greater in, and purchase a phone through Costco; You get $1000 in rebates per line/ phone.
Example: I just transferred to AT&T from the sprint deal; I purchased two refurbished 32 GB iPhone 7's from eBay for $225 total, traded those in for $500 each; ported my two lines from Sprint to AT&T (this part of the deal is not advertised because of Costco's policies) this gets you $200 for each line; purchased two iPhone 11 Pro Max 512 GB phones, this gets you $300 per phone; and sign up for autopay, this gets you $10 off per line; My service plus phone payment is $163.00/ month for 30 months. All of the discounts are built into the bill. Essentially, you get 2 iPhone 11 pro max 512 gb phones for a total of $500 each and you pay $129/ month for unlimited AT&T service.