If prices are denominated in fiat and bitcoin payments are not the only payment method available, then refunds should be in fiat.
If prices are denominated in btc, and btc is the only method of payment provided, then refunds should be in btc.
Ok well which is it?
Which prices are given on the website? I'm at work, and anything bitcoin related (apart from btctalk, weirdly) is blocked.
Prices are quoted in FIAT
Payments are either FIAT via Bank Transfer or via Bitcoin
So it would stand to reason that if you paid for a $10,000 item in Bitcoin you would get back $10,000 worth of Bitcoin
and if you did a bank transfer of $10,000 you would get back $10,000 via a bank transfer.
If prices are quoted in fiat and bitcoin is only payment option, I'd consider the fiat price to be a fair return.