Processing Fee based on Vendor by Tanner Agar

I have a new vendor on my site and will need to charge a processing fee for orders containing their products. 

Essentially I need an if then statement which says: If product vendor equals ABC then add $5 fee. The $5 is flat regardless of having 1-100 ABC products in their cart. 

There doesn't seem to be a way to do this, but I have to have it. The only alternative is to play some strange pricing game on ABC products which isn't satisfying to myself or my vendor. 

Any idea how to get this into the code for the cart? Thank you.