Which 'Discount type' are you using - Assigned to Products, Assigned to Order Total, Assigned to Order Subtotal?
The Fixed By Weight By Total plugin will calculate the sub total as the sum of cart item prices (including item/product discounts), but does not seem to account for discounts assigned to the order. (Probably a short-sighted decision because of how it had to deal with items-per-warehouse shipping.)
It would need customization, or Shipping Director
can do it using one of its variables:
$SubTotalWithoutDiscounts (uses orderTotalCalcService.GetShoppingCartSubTotal() )$SubTotalWithDiscounts
$ProductTotalWithoutDiscounts (uses PriceCalculationService.GetSubTotal() )
Your rules in Shipping Director would be:
[$SubTotalWithDiscounts] > 20.00