Who is going to subsidies that discount? After all, if you can buy an car for a discount with bitcoins, who's
going to cover that spread?
I'm not sure how BTC has low fees and quick transactions, at least on the buyer side. I could pay with a debit card and get charged nothing extra and the transaction is instant.
On the seller's side, I'm guessing you get charged somehow? But exchanging BTC to fiat and then withdrawing must have some fees.
IIRC the seller will typically pay ~3% fees for credit/debit, which is usually more than BTC/USD exchange fees. This fee is invisibly added to prices, that's why some things appear cheaper when priced in BTC.
The transaction isn't really complete until the buyer runs out of time to issue a chargeback, which can be up to 120 days.
http://www.dailyfinance.com/2012/07/31/credit-card-chargeback-merchant-disputes/
If you're a merchant, Bitcoin trades the risk of chargeback (which averages a loss) for exchange rate volatility (which averages a gain).