Shared
It’s 40 keys for a pack of 3. One goes in each slot. You can mount up to 3 simultaneously (i.e. reduce firepower lost by 1,2, or 3 units)
If it’s auto-use then you can have no more than 1 unit reduction per death, which doesn’t seem worth it.
Making it perishable allows for greater reduction in power lost. If might seem overpowered because it applies to all deaths in a mission, however if you die a lot you are unlikely to be performing well anyway. Plus, as a perishable you lose it at the end regardless, so there’s no benefit in stocking/mounting units “just in case”.