We don't charge more. Bitcoin has an associated transaction fee that we do not control. You also have to use bitpay's exchange rate which may differ slightly from other exchanges (see: https://bitpay.com/bitcoin-exchange-rates). At the end of the transaction we only receive the amount we charge ($8 in your example).
I've already stated it in another thread but we are actively working on providing alternative methods of payment in the future. The last thing I want people to think is that we are being deceptive with the payment methods we offer. If you are unhappy with bitpay's exchange rates or Bitcoin's transaction fee's then it would be best to hold off on purchasing until our alternative payment methods are ready.