If they payment is request in USD, Then just think about how you have so many more possibilities of "refunding" your money.
IE you payed with CC/Paypal USD=Refundable
If you payed with the Glorious Bitcoin, then you cant Refund on your own, You need them to do it for you, and they will have to to a conversion rate and hand you back equivalent USD.
So why not just stay on USD?
I highly doubt they would calculate conversion amount into the equation, they have no control over the fluctuations in price, and would make no sense for them to convert from a business standpoint.