I remember last year DS had a USAFA LoA and a few weeks later received a postcard saying he was an official candidate. It seemed a little redundant to have an LoA and to get a postcard saying that he was an official candidate. You can probably do a search to look at last year's posts, but I do remember other people saying that they had received the same postcard.
SD Academy Bound, at this point if you have an LoA, basically the only thing you really need to worry about (unless you have additional conditions on your LoA) is the nomination. The postcard, if I remember correctly, is a mere formality.