Free Shipping Coupon Display Total Wrong

I have enabled a free shipping coupon code and it works correctly when the shipping value is less than the product value. When the shipping value is greater, the sub-total shows just the shipping cost, when the actual cost should be the lower product cost. I am using flat rate shipping.

This problem is just with the display of the subtotal, i have went a step further to PayPal screen and the correct subtotal is displayed.

https://www.ads-software.com/plugins/wp-e-commerce/