Depending whether you were required to pay postage in the first place, you could have saved money on postage by going to the Amazon store and having them print out a label for free. If you have a postage-paid label already, if they can't accept it for whoever the courier is, in any case they would be able to accept the return and they could provide their own label free of charge if needed.
If it's a USPS label that requires postage, you must take it to the post office. If the label doesn't require postage, you can drop it off.
Thanks! Your answer is awaiting moderation.