so in the add to cart button, you are setting the DiscountGroup column to use the session variable?
you have 2 options:
1) Use the session in the calculation instead of the discount group column, for example:
abs(isset($_SESSION['DiscountGroup']) && $_SESSION['DiscountGroup'] == 'A')
2) Create another calculation that sets the DiscountGroup column value to the session:
abs(isset($_SESSION['DiscountGroup']))?[DiscountGroup] = $_SESSION['DiscountGroup']:0