Again - sorry - I am not so sure about the parents knowing. First purchases was May 10. I think it was a few hundred. That bill wouldn't have come until say, June 1. Then due at end of June. (of course, depending on the cycle of the card)
Most purchases were made later in June, early July. None of the totals we have seen so far have been more than a few hundred at a time.
He could have made payments with his stipend money, since he wasn't using it for anything other than rent, utilities and food. He wasn't partying, socializing, doing fun things - and his only hobby that we can gather was putting together a plan of mass murder!
I don't see this as anything to raise the flags for the parents.