I thought you could order AppleCare+ any time within 30days of purchase (i.e., doesn't need to be ordered at time of purchase of iPad).
You can technically, but it's easier to just order it with the initial purchase. The caveat is, (and makes total sense), you have to go in to an Apple store to have your device inspected when purchasing Apple Care after the fact. That way you can't break your device within the first thirty days, THEN purchase AppleCare to get it replaced.
Although there are companies and distributors that don't have this requirement for post-purchase of extended warranties, Apple does.