A per diem charge is the fee the ocean carrier charges for each day past the number of “free” days that the container is away from port. Per diem is also known as detention.
The ocean carriers are eager to get their containers back so they can keep them in circulation. The per diem charge discourages importers from storing containers for long periods of time.
If you need to store your cargo for a longer amount of time but wish to avoid per diem charges, you will need to have the container unloaded at a warehouse. The storage charges at a warehouse are significantly cheaper than per diem charges, and the trucker will be able to return the container back to the carrier on time.