That alone is currently a significant volume to liquidate. Enough to move the price 4-5% on some exchanges. Bitpay only takes a ~1% fee, but I assume their exchange rate is padded to absorb some of that risk. They may also sell bitcoin off the exchanges, which doesn't have a direct impact on the price.
PS. When I order from Amazon, etc. I trust them. I get 2% back using bitcoins (from Coinbase.com (-1%) -> Gyft (+3%)). I like the 2% discount more than the credit risk of Amazon not making me happy at the end of the day : )
It is kind of nice having a currency that appreciates in value over time vs slowly getting robbed by inflation or capital gains tax.
The USD price is locked in, and they never need to hold bitcoin. It is just a cheaper payment method.