Did you open a dispute in the resolution centre at the bottom of the page for not as described
YOu have to do that for all disputes and if the item is not as described then the seller should send a pre paid label for the return of the item
If it was change of mind then you pay the return
If you dont do it through the dispute and pay the postage yoursefl then you have no way to force the seller to refund that money
Ebay cant force it either.
