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.
One thing to consider is that the exchange API's misbehave, there needs to be a way of mitigating for their misbehaviour, but not via means that misinterprets huge rapid price movements as something to be ignored. I think that using the right length of price average probably represents the best compromise, maybe using something as long as a 3-day average would smooth it out well.
Anyway, the message seems to be that Black Arrow should alter their super-slow BTC payments, as it's causing confusion and uncertainty amongst potential buyers.
Indeed. I know they are well aware of the problem - I spoke with them saturday about it. I will be sure to bring it up again.. because if they find a good solution I can "borrow it"