So the seller would get the item back, but be out of pocket for the original postage cost, due to your error.

I accept you have a grasp of postage costs with experience in a PO.   

I have sold hundreds of pairs of boots and from experience posting Australia wide, $43 would be inline with standard postage both ways.

In any event, the return is done and dusted, so unless you can come to some arrangement with the seller, your only option is to on sell them.