How to calculate and rollup the value of currency fields in a sub-item

I am using the Sales CRM template which I expanded on. In my case, I have deals/opportunities that may have more than one services or products, each with different $ values and % commission rates. In order to capture the products and services, I am using sub-items in every records with the $value and % commission for each.

I was trying to use a formula field in the sub-items but that does not seem to be possible. So, I attempted to use a formula field at the record level, where I could calculate the $value * % commission = $value for the product or service. The the goal was to add via the Sum formula the total $value for the product or service.

..I am stuck.

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