How do I allow a free product into a cart, but only if cart net value is above $10?
Take a look at the Ubercart Restrictions module.


  This module will do its best to prevent the wrong sort of people from
  ordering the wrong sort of goods, or to ship them to the wrong kinds
  of places.
  
  For example, some products cannot be ordered by minors, or some
  products cannot be shipped to certain states