Well if that player is not online to hire those mercs or those mercs are not online, etc then how does it get defended. Yes, I would say allow the player to hire mercs to defend it but its always up in the air with some corps where a solid PVE + mercs backing it up would be more beneficial to the EVE player.

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Customs Office stats
Shield: 10,000,000 HP
Armor: 2,500,000 HP
Structure: 2,000,000 HP
1 reinforcement cycle (activated on shield <= 25%)
Reinforcement duration: 24 h + time until variance.
Reinforcement Variance: 1 h
Shield will be 0% after the reinforcement mode ends.
Once it comes out of reinforced mode, it will not enter reinforced again unless it is repaired above 25% shield
A POCO in reinforced mode cannot be accessed or utilized
InterBus Customs Offices have no reinforcement mode.
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Notifications

When your Customs Office's shields fall below 99% you will get a notification so that you can defend it
When your Customs Office enters reinforced mode you will get a notification that includes who put it in reinforced and when it will exit reinforcement mode

Destroying Customs Offices

In the Crucible expansion, players can destroy Custom Offices in Low-sec, Null-sec and Wormhole space. Should a Customs Office be destroyed, then a kill mail will be sent to the owner and party laying the final blow.
Corporations can edit re-enforcement timers in order to ensure that any Customs Offices that are under siege can be defended at the time of their choosing.