If you want something from Amazon right now, you're not losing money, you're gaining. spendbitcoins.com doesn't mark up the prices. Right now, they offer a value of $10.22 per bitcoin, each of which you can buy for 50 cents less at MtGox. In short, you can currently buy a $100 Amazon gift card for $95. If enough people do this the mtgox price should converge to the daily weighted average.