setting a negative shipping amount based on variables
I want to offer free shipping if a session variable = a certain value
if there are more than 2 items in the shopping cart.
However, I also want to offer free shipping if a different session variable exists
AND there are more than 3 items in the shopping cart.
I don't think I can add all of these to my baserate and increment rate shipping . I'm thinking I need to have a different shipping method that sets a negative amount of shipping if these variables are met and that negative amount will calculate to zero.
My rate charged is based on a flat rate of : $_SESSION['BaseRate']
How do I make this amount ... $_SESSION['BaseRate'] ... a negative amount?