I tried the store but was told no so I came home and called support. They emailed me a label that I taped to the box that my 6S Plus came in along with the original box my 6 Plus came in. I dropped it in my mailbox and its on its way. I should have dropped it at the post office and asked for a receipt but we'll see what happens.
The rep I spoke to said he didn't have a way to send me a label. Is that the only way they can tell the phone came from you? Also do you mind saying what you account status shows for the phone you are returning?