Nothing is more frustrating than mis-clicking a vendor and buying something you didn't intend to with your hard earned currency — and then realizing there's no way to get a refund.
Blizzard has responded to feedback about players accidentally purchasing already-known cosmetics, leaving them unable to refund for the purchase they intended.
As of now, players who purchase an already-known cosmetic from the Midwinter event are unable to refund it, meaning those spent are gone for good.
With the exchange rate of event materials for proofs being rather high, it makes sense that players are unhappy about this — one back trophy could have represented quite a lot of farming time for that player.
Thankfully, Blizzard seems to have heard the feedback, so hopefully a refund option will appear in the future.