The rules 'Add a product to the cart' assumes that you want a 'product' line item to be created. What if you want to add a specific line item type? How can you do this through Rules? Only way that I have fond so far is to use the Entity Rules to create a line item Entity of my desired type, although it when I do it that way it doesn't seem to get associated with the Order correctly and doesn't show up in the cart.
It would be good if the 'Add a product to the cart' Rule would let you specify what kind of line item should be added.