Need method to discount (reduce) shipping charges, ideally based on a related value in PaaS DB

We have a use case where we need to offer a % discount on shipping charges in cases where the customer order occupies 26 feet of the truck the product is shipping on.

Order footage is a value that's being calculated in a PaaS extension we use for order entry and ideally we'd like the discount automatically applied if the calculated footage meets the requirement, however I'm open to keying off something more manual as well. Currently we offer a "Cost of Truck" discount for full truck quantity by creating a credit against the invoice, we need something that is more automated due to the increased number of orders that will meet the smaller criteria.