If you haven't read through, I'm a WSE for Wireless Advocates who operate the kiosk inside of Costco, and I'd like to help clarify this for you.
First thing to understand is that we piggyback our carrier's return policies. Verizon and T-Mobile give you 14 days to return the phone you purchased from us for either a diff phone, or to go back to your old phone (the one you're upgrading FROM) and get your upgrade back. Both Verizon and T-Mobile allow you to return the phone for the same phone for up to 30 days in the case that the phone is defective. This allows you to get a phone you know is brand new, and not a refurbished phone. AT&T will give you up to 30 days to exchange the phone for a different phone or to go back to your old phone and get your upgrade back.
What Costco does is extend the return policy from 30 to 90 days. You can exchange the phone for the same phone in the event the phone you purchased from us has a defect. If you simply want your money back, yes, you can for up to 90 days. However, AFTER 14 DAYS YOU WILL NOT GET YOUR UPGRADE BACK. Therefore you can return the phone and get your money back, but if you try to buy another phone you will pay full unsubsidized price (not the upgrade price) for it.
As a Verizon customer, you will have to satisfy 20 months of your contract before you can upgrade again. So if you return the phone on the 90th day after purchase, you will get your money back, but you will NOT be able to upgrade again for 17 months after that.
I hope this helps you to better understand. I'll be glad to answer any other questions you may have.