Buy from AGEOD, it's cheaper (27$) and easier.
You click here
You download BoA.
You download and apply the last patch.
You play.
Later if you've uninstalled BoA, or if you buy a new new computer you can just download BoA again (unless you saved it on a CD), enter the serial key and you play.
As Pocus mentioned, AGEOD stores your serial key in case you lose it...