iWatch Trade-In

Hello,

I am trying to trade my older iWatch 3 series in towards a new iWatch 6 series and thought I had cleared the 3 series before shipping it in, but several weeks later Apple sent the watch back saying it was still linked to my account and they couldn’t open it. How do I make sure it’s officially cleared before I send it in again? I already have the iWatch 6 that I’m using.

Thank you!