HandlingTotal added to OrderForm not included in total cost for cart

Description

Trying to use the field HandlingTotal on the OrderForm to add cost for orders paid using an invoice. Adding the cost to the orderform works fine but the cost for HandlingTotal is not included in the Total cost returned by the OrderGroupTotalsCalculator.

Steps to Reproduce
(tested In Quicksilver version 9.24.1)

1. Add code to the Index function of CheckoutController that adds HandlingTotal to the OrderForm prior to calling CreateCheckoutViewModel. That code looks like this:

Cart.GetFirstForm().HandlingTotal = 99;

1. Add an article to the basket.
2. Go to the checkout page, forcing the addition of a handling cost.
3. The orders summary total displays a total of the article plus the cost of the selected shipping method. It does not include the HandlingTotal cost.