@Kevin_C , thank you for your comment, however, as mentioned in my post, I had already enable this feature in my account settings. The single income account I picked to track the expenses is the "Sales of Products Income" account. The problem is that when I run a report on this account it is empty, regardless of the date range I select. 

I already have a good amount of expenses tagged as billable and categorized to the Cost Of Goods Sold expense account. Why isn't the income account I selected tracking the billable expenses?