Everyone is assuming that he wrote all this and mailed it in that two hour time frame, but as you noted, he could have dropped it off well before the shootings, and it only got logged into the system in that two hour span. Otherwise, someone might possibly have seen him moving around, going to the FedEx drop off point, and I haven't seen that mentioned anywhere.
There was a Postal Inspector at the news conference that said that it was dropped off at the Post Office at the time it was stamped, and the postal employee recalled the shooter dropping it off.
I was certain in the news clips I watched that they were showing a “fed-ex” type package, but maybe they were and it wasn’t the package HE sent, (typical TV journalism using “file footage”).
I was also told by someone else here that they know he handed the package to a mail person who stamped it for time. So I guess he did run off to the post office on his way to do the deed.
I don't think anyone assumes he wrote all this in that time. It would be logistically impossible, even without considering the time it took to shoot, edit and burn to disc the photos and video clips.
The more plausible scenario is that he had his whole package, which he'd probably worked on obsessively over a span of days, put together and sealed in an envelope. He shot the first two victims, then sent it -- maybe stopping at his dorm in between to pick it up or change clothes.
Then he went to the post office to mail it. It's a kind of symbolic act -- he was committed, he had crossed the Rubicon, and there was no turning back. Two were dead, more would die, and he would not live to see the end of the day.