I agree with Purple and vicki, but not Faldred! (jk).
DVC points reservation people can (or at least could) pay for their plans upon check in, but it still needed to be added no less than 48 hours before check-in. Cash reservation people can add it no closer than 72 hours before check-in, and it the follows the rules of all Disney package payments (due by 45 day mark, any changes after that mark must be paid for on the spot).