returns label for post office but parcel too large for post office

[pawj.p.ufj3xh3]

I have requested to return an item. Ebay has sent me a post office returns labels but the item to return is too large for the post office to accept. I seem to be in a battle with the seller who says I need to speak to ebay but no way of contacting ebay other than by phone.

Any help appreciated.