Amazon has weird limits on how many boxes you can buy at once. for instance, I was limited (previously) to 3 battle chests for my 'account'. On that link, it seems to be limited to one WOTLK box...dunno why. Nasty Gram from amazon follows:

We are sorry. This item has limited purchase quantity. Because you have purchased the maximum allowable quantity in the past you cannot buy any more units of this item.