Here is what’s inside the shopping cart. First you see the two main domain-object projectors: the multi-column list box and the total amount.

The component at the upper left manages the collection, which is the list of items in the cart.

The new component at the bottom sends a self-revealing message to the list, which then computes the sum of the ExtendedPrices of the elements of the collection. Note that there is no loop programming.