setting a negative shipping amount based on variables

I want to offer free shipping if a session variable = a certain value
AND
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?

would it not be easier to set this up as a discount if the criteria are met. so that the level of discount matches the shipping cost.

in your cart display, the shipping would still be shown , but the 'free shipping' discount would also be shown, with the cart subtotal calculated correctly

My shipping is set to "baserate" for the first item and "incrementrate" for each additional item ... How could I do that for the incremental rate?

can you show me the shipping calculations you have set up for baserate and increment rate ?

2 Calculations for shipping:

1) Baserate
if TotalWeight >0
calculation: Flat Rate = $_SESSION['Baserate']

2) Increment
if TotalWeight > 1
calculation: Total Weight times $_SESSION['Increment']
minus $_SESSION['Increment']

try putting the exact same calculations in the discounts tab of the ecart object, they should calculate the same, but as negative values, thus cancelling the shipping