Trade rental charges operate differently from those on skip orders, as they have an additional option.
Note: This will only be true if the Unit Of Sale on the contract item is RENTD, RENTW or RENTM.
Go to the contract item in question
Click the Edit button
Check the "Create invoice Data" option
If this is enabled, then this item will be picked up and charged when you do a trade invoice run that includes this contract
Check the "Periodic Charge" option
If this is enabled, then this item will generate a rental charge when you carry out a Self-Contained Charges run in the Trade Module.
Warning: If both of the above options are enabled, then the item will produce 2 charges - open on the Trade invoice run and another in the self-contained charges run.