Any service that converted between BTC and USD would probably require at least 1 confirmation - i.e. it would take 10 minutes before the USD were sent to your account.
The market value could change in that time, perhaps by 10% or more when the market is unusually volatile. However, assuming that these rises and falls average out over the long term, a web service could guarantee the quoted rate.
So the user would buy the product using $20 worth of bitcoin, which would manifest itself as $20 (minus a percentage fee) in the merchant's account within 10 minutes. Any hit (or gain) due to fluctuating exchange rates would be taken by the web service sitting in the middle. Over time the fluctuations would average out - at least that would be the hope - or at least covered by the transaction charge.
The market value could change in that time, perhaps by 10% or more when the market is unusually volatile. However, assuming that these rises and falls average out over the long term, a web service could guarantee the quoted rate.
So the user would buy the product using $20 worth of bitcoin, which would manifest itself as $20 (minus a percentage fee) in the merchant's account within 10 minutes. Any hit (or gain) due to fluctuating exchange rates would be taken by the web service sitting in the middle. Over time the fluctuations would average out - at least that would be the hope - or at least covered by the transaction charge.