The ability to edit default charges per unit on Recurring Charges for shipments is set in the Shipments and Scaffolds tab of the Global Options dialog.
To enable editing of recurring rent default values on Shipments:
Click the Tools menu and select Global Options.

The Global Options dialog appears.Click the Shipments and Scaffolds tab.
Click the Shipments sub-tab.
Click the Allow editing of recurring charges checkbox
Note: This enables editing of recurring rent charges on Shipments.
Click OK.