Enforce Category Setting in Taxes & Calculation Rules PER BILL

[saviB]

I need a piece of software or code, to enforce the Product Category setting in Taxes & Calculation Rules when the Type of Arithmetic Operation is set to any of the per bill options, specifically Price modifier After Tax Per Bill. Deliverables include code block(s) and file location / line # to insert.

End goal is to add a dollar amount to an order if a Product of the specified Category is in the cart. That dollar amount is per bill - not per product, meaning it is allways one amount regardless of the quantity of the product purchased.