Right now most Military Shipments can be sent only once until Imperial age where they get unlocked for one more use.
I would like to suggest adding new Challenges/Achievements that add one more use to each one of these military shipments allowing them to be shipped twice or even more times as more challenges are met.
So each challenge that is met will unlock one more use for one of the military shipments.
There are many military shipments in the game. Therefore, the number of new challenges can be high.