Any merchant who accepts Bitcoins needs to cash out really fast. We're seeing 25% changes in a day, and 10% changes in 10 minutes. By the time a Bitcoin transaction is confirmed, it may have lost 10% of its value. You can't run a business on that.
Calculate with +3% lower rate, then, right after payment, automatically try to cash in the money at mtgox. If success: all is good, ship order, if unsuccessfull: tell user you can't sell at that price any more, offer new price or to abort order or return the BTC.
How is that easier than using a credit or debit card? The merchant would have to wait up to an hour to have the bitcoin confirmed, and then maybe another hour to transfer it to MtGox. If the price has moved down in those two hours, email the customer to demand more bitcoins. If the customer refuses then wait another couple of hours for the bitcoins to be returned to the customer. It's creating new problems while solving old ones.