Yes
is it the return label that has the "postage required" printed on it ? if so, then yes, you will need to pay to have the item returned to the seller................for the correct amount of postage, you will need to go to a post office and have the item weighed ( unless it's small enough to fit into a standard envelope )
Yes, you will have to pay for the stamp and such to have the package shipped. If it is larger than what could go into a mailbox, the it may depend on weight and size.
Yes, you pay return shipping. They have already paid to send it and it might have been bashed about by the courier.