Creating an option to add a fee to purchases

I would like to see an ability to add a fee to an order if a customer is ordering less then the minimum amount of a product. For example: My suppliers charge me a $50 fee if I put in a order that is less then 10 embroidery items. I would like to let my site visitors know that an extra fee would be applied to those orders, and have the option to apply it automatically. How difficult would it be for me to add that kind of feature to the site?