yes you have to give full refund including original postage.even if buyer is stating this falsely (unfair yes i agree but they are the rules) BUT do not refund any money whatsoever until you have received the shoes back. buyer must send them back AT THEIR OWN EXPENSE. they must send them back via recorded or special delivery so that ebay has a tracking number. its not enough for them just to have proof of postage they must prove the shoes have been actually received by you.even if the buyer opens a dispute they will still only get their original costs back. you will not be made to refund return postage.
of course if the buyer was correct in stating there was a problem with the shoes which hadnt been pointed out then you of co0urse should refund return postage as a matter of courtesy.