They can probably tell on the postmark what ~time~ it was postmarked, and they find the time interesting, because it appears it was mailed between the shootings. That in itself is interesting.
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.
If it was mailed at a post office, or through one of the newer postal vending machines, rather than a postage stamp it would get a sticker showing the postage, the date and a barcode for the destination. I think those have the time on as well. The vending machines that you use credit cards for you can mail parcels at, and they use the same kind of printed postage.