I mean, obviously Bitcoin is a pretty silly way to buy a game on Steam, but this seems to be 100% user error.
...well, probably. There's some key info missing here, but this seems highly suggestive:
"BitPay emailed me saying I underpaid by 3 cents at the time, and said to refund me. I checked the price of bitcoin 0.003853 btc in usd is around $31 which was a lie on their part."
No, they said he underpaid by 0.000004 BTC. And the number he was googling is, coincidentally, 0.000004 BTC lower than the transaction value in his screenshot, so...