There are two ways for an admin to award prizeboxes.
One is using the Award Prizebox tool in the user center. This tool lets you award prizeboxes to a user AND sends an automated PM.
The other is just by editing the user and adding the prizebox MiniRank to their account with a count greater than 0 (or adding to the count they have). This does NOT send an automated PM.
I think the issue is that the award prizebox tool is more restricted than the user editing functions, so I'll have to adjust the permissions so people can access the prizebox tool. Once that's done, they can award them with an automated PM. (This only applies to prizeboxes though, for other MiniRanks there is no PM)