Need help and willing to pay for Modified Shipping

I would need the shipping to modified with a set % of 18% Delivery and Handling Charge.

There is no handling charge for Orders being Picked up. But for Catering there will be a 18% handling and setup Charge.
Can someone help me by setting this up please. And how much that would it cost.
robert

http://broadwaybakerycafe.com

Click on the Catering menu and tell me how much the cost be please