I think it's just a case of using the API on the exchanges to calculate a weighted (or otherwise) average for yourself. Maybe not such a small task, but it would be better to get it done, as it only a case of keeping up to date with any changes in the exchange API's to maintain the system. Then you can take the money at checkout, and the customer actually reliably knows how much they are paying.
See im a hardware guy, not software. I will pass your suggestion on to my web dev team tho... I hate calcing it my hand and a Automated system is what I want.