Begin by selecting the "Inventory" button from the menu at the top of the page.

Next, select "Services" along the menu on the left hand side of the page.

Find your delivery services (in this example, "Standard Delivery") and then click on the three dots (. . .) to bring up options to edit.

From the dropdown menu, select "Edit Service". 

Finally, scroll down until "Mileage Rate" appears in view. Edit the cost per mile to suite your needs.

Don't forget to "Save & Close" before exiting.