Hi Mark, do you want to add the handling fee to individual products rather than all of the items in the cart?

If so, you could do this with Commerce Product Add-on. This module will let you create a new product, in your example this product will be called 'Handling fee'. You can set a price for this and this will be added to your products. You could make this a required field if needed, so buyers must select this.

Theres a screencast available for this module on this site where they used a product called 'Donation' to add a fee to the product being purchased.

It might help you out in achieving what you need.

Terry.