How to Create autopopulating Discount Item.

I want to create a feature like Positive Discount (%) in OM so that when I raise a sales order for a particular inventory item, the postive Discount line autopopulates as a % of the item price.
This autopuplated line should have its own revenue account in AR. This line is an insurance line where if a customer buys a mobile device or accessory wiht a particular inventory item code from us, the insurance line autopopulates.