For a return the usual return label sent is for a 2kg item. If one for a heavier weight is required, you needed to contact eBay Customer Services, and they would have provided the correct label, and charged it to the seller's account.
As you have now paid for the correct label, there's nothing eBay can help you with, and you need to message the seller for them to refund you this cost, but nothing you can do to enforce them to do so.
@c4rpf