Once a parent's card is saved (from their first manual payment), every later postpaid invoice auto-charges automatically — but the grace period you've set on that billing plan still applies even with a saved card on file. If your grace period is, say, 3 days, the auto-charge fires 3 days after the invoice date, not immediately. Set the grace period to 0 if you want the charge to fire the next day after the invoice generates.