Enable Unit Cost on Opportunity Details

I’m trying get the unit cost on the opportunity document details.  I’m following the same process I was given for enabling the unit cost on sales order lines. (https://community.acumatica.com/customizations-147/how-can-enable-the-unit-cost-field-on-the-details-tab-of-sales-order-6709?postid=21524#post21524) I’ve added the new workflow with the unit cost field and the row selected event handler. The OpportunityMaint doesn’t have a definition for  Base.Document and Base.Transactions but I’m not sure those are necessary since I removed that code from the sales order line and it still has the unit cost enabled.  

When I add a line to the opportunity document details the unit cost is editable but then when I select an inventory item the unit cost is no longer editable.  I’m not sure what I can do to change that behavior.