If the product was priced in USD and you were using BTC as a payment method, it would make sense for the refund to be of equal USD value; but it's completely up to you and the seller, really.
I've never used CrypoThrift so I can't comment on how they will react, however we have had similar situations come up with our store.
We automatically exchange the majority of our BTC instantly. Only our profits are kept in bitcoin, and unfortunately this will remain until our overheads can be paid with it.
If a customer purchases from us and changes their mind, we can only refund them a BTC amount based on the USD value they paid. This is to protect us from speculators as well as exchange rate risk. We will not take a loss because a customer changed their mind. For the most part customers have been happy with this once we’ve explained the situation.
However, if a customer purchases an item and we cannot ship it then that’s a whole different story. Why should the customer be out of pocket if it is our fault? In this case we will send back the BTC amount regardless of the exchange rate. This is done on a case by case basis however so don’t buy 100 of the oldest product you can find on our store!
Hopefully that assists you. I personally think that if the vendor is at fault then you should be repaid your BTC in full. Realistically however it’s going to be up to them and CryptoThrift.