Multiple item shipping discount

Hi there,
I have products that cost $3.00 to ship. I entered $3.00 in the Shipping Basics Flat Rate. If an item cost more than $3.00 I then add the additional amount in the Item Based Shipping of that particular product.

If a customer adds more than one item to their cart,
I would like to offer a discount on their shipping.

Example:
Item A) shipping $3.00
Item B) shipping $3.00 additional

How do I execute this scenario instead:
Item A) shipping $3.00
Item B) shipping $1.50 additional

using ShopperPress Version 7.1.4

thanks, Laurel