You may follow the steps in this article or you may use the Guided Tutorial right in your OMS called "Discount Shipping on Nexternal Product - No Coupon" for the steps to purchase Endicia postage. (Guided Tutorials are accessed separately from the Support Center in Foundry, through the link to Guided Tutorials in the system navigation under the question mark at the top. In the legacy standalone version of Nexternal they can be accessed through the Guided Tutorials link in the left navigation.)
Here are the steps:
Navigate to Products and then click on the name of the product for which you wish to offer free shipping or discounted shipping.
Click Edit next to Product Overrides.
On the top right, click "Override Shipping Rates", and this will reveal new Shipping Rates fields below the overrides section.
Enter the rates you want to charge next to each method in each zone. Then click Next.
Tip: Only fill in rates of the methods you want to override. Leave all others blank. (The regular shipping tables in Settings will continue to apply to all blank methods. Only the methods for which you enter amounts here will be overridden.)
For Fed Ex, remember that both Home and Ground are ground methods.
To see the zones and methods as they are set in the shipping tables in Settings, click "Show Zones & Methods" to the left.