To add flat rate mileage or kilometrage based surcharges go to Pricing.
Scroll down below the pricing and click the green plus next to Surcharges.
Enter your description and select 'Flat mileage amount' or 'Flat kilometrage amount'.
Here is an example of bucket pricing. It is important to include mileage down to the .01 to make sure all mileage is covered and priced correctly.