It might be that Holmes sent the package to someone at the university hoping that he could somehow make the university culpable financially, as in lawsuits against the university for not taking steps to stop him. If he had recently failed an important exam he might have been angry and wanted to strike out at the school and administrators.
He carefully planned the whole thing, including booby-trapping his apartment, to kill or wound as many people as possible. If that was his motive, he is just plain evil.
He did send a notebook on July 12, to a professor at the university with drawings and details of the attack. Unfortunately the notebook was discovered after the attack. I understand it is not certain if it was his professor or not, but if the notebook had been discovered before the attack it could have been prevented. Also it was reported this morning, that he wasn’t as smart he was thought to be and the reason why he dropped out was because he failed