I agreed to send old phone back but can get more with trade in

[Netties]

I have just upgraded my daughters phone, I was offered to send old phone back to clear what's left to pay.  Anyway was taken to sky trade in and was offered more, but I'm confused.  Do I use sky trade in or just send old phone back.  There is a £59 difference in my favour.  What and how do I do this?

if anyone has any tips or can advise would be greatly appreciated 

[caesarome]

If you are being offered £59 for the value of the phone then that will be used to pay off whatever is left on it. The device itself needs to be returned to Sky so when you do this make sure you get and keep proof of posting. There should be a label on the inside of the packaging her new phone came in so you can use this packaging to return it.

[Netties]

Apologies I wasn't very clear, to upgrade they have said the phone is worth £276 and there is £276 left to pay which would bring it to £0 so I wouldn't have pay anything.  Sky trade in popped up and has offered me £325 so I could potentially pay the £276 and would benefit from £59. 
I will try and contact someone today as I don't think it's fair that you only find out after checkout that you can use sky trade in.