You would have to offer a discount for paying with bitcoins for this to work. I know a lot of merchants (in real life, not online) that offer small discounts to people who use cash so that they don't have to pay credit card transaction fees.

In any case, if you accept online payments via credit card, there wouldn't be much point to converting them to bitcoins after the fact, unless you actually prefer to hold BTC instead of USD. You've already paid the transaction fees.