Seems fortunate that the mail was picked up in that two hour window and ‘timestamped’, as in processed. Something seems fishy about this story. Although there may be a mail office on campus which means he had to have it weighed and postage paid to a person behind a counter.
A guy murders a couple people, goes back to his room, writes a long rambling letter and mails a package to NBC before murdering 30 more people, all before lunch. The guy with the "fishy" behavior appears to be Cho.