Intended to make bulk discounts and surcharges that display between the Subtotal and Total easier to apply, the Commerce Bulk Discount module provides a way to add custom price components to orders without using PHP. Once an order bulk discount price component has been created, it can be implemented by creating a Rule triggered by the 'After updating an existing commerce order' event. Logic can be applied via Rules conditions and the bulk discount amount calculated via a Rules action. The calculated amount can be passed into the provided 'Apply bulk discount to order' Rules action which displays the custom discount or surcharge between the order Subtotal and Total fields. See the screencast link below for a demonstration of how to use it.