Legally, or Paypally at least, the buyer is required to return the item to you at their expense.

Think on it. Does your average Big W or Kmart refund your time and petrol costs if you take something back for a refund? No.

AFTER you get the item back is the time to decide whether or not you refund the return postage. Paypal will refund the total cost (including postage) to the buyer if the item is SNAD. It's up to you to determine whether or not you inadequately described the item.