You can use multiple shipping routes only with SuitePromotions that have shipping impact.
Multiple shipping routes are supported when:
The Multiple Shipping Routes feature is enabled
The Enable Item Line Shipping box is checked on the Items subtab of the transaction
The SuitePromotions feature is enabled
If the above criteria are met, when an eligible shipping SuitePromotion is added, the shipping discount is applied after you calculate the shipping costs.
Shipping SuitePromotions can only be automatically applied after shipping costs have been calculated.
If the SuitePromotions feature is disabled, NetSuite displays a message confirming that promotions with shipping impact are not allowed when the transaction has Multiple Shipping Routes activated. The promotion is removed from the transaction.
For more information regarding Multiple Shipping Routes, see the help topic Multiple Shipping Routes.