I find the ingenious idea of being able to receive by mail the code in less than a minute to be able to buy the xbox website of music, games, extensions ... like this kept secure by need to go to the site Microsoft xbox are credit card number that prevents his card piracy problem. In addition, we can choose according to our needs amounts 5,10,15,20,50 euros.
Be aware that if you buy in the wake eg a xbox live card or other amounts 50, there is an audit by Amazon to avoid that someone is able to retrieve your information for you and buy another map, I find this very smart move by against, control may take 1 minutes to 4 hours (for me this has never exceeded 5 minutes maximum.