Unwanted cell phone return

We are trying to return an unopened new cell phone that we do not need or want.  This is part of our effort to disconnect phone service and only keep internet and tv.  We talked to Felix on December 3 and he issued us a refund for the phone.  He told us to return the phone to our local Xfinity Store.  I went in last week and was told that since we had the phone in our possession for more than 14 days, they could not take the phone back, and instructed me to return via mail.  To whom do I send the phone and how do I get a UPS mailing label to make that happen?