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.

There is a difference between buying online and in a B&M. A buyer has the opportunity to inspect physical goods in a shop, so obvious faults are often picked up prior to purchase. Of course that is not always the case but there is a lesser risk of buying faulty goods in a store.

Secondly the buyer has to return the parcel to the post office, perhaps buy packaging for it, line up in a queue to post it etc. This would be similar to taking it back to a store. With a B&M though there is no additional outlay of cash for postage.