Discount by shipping provider plugin is a discount requirement type plugin. It allows to assign a discount to customer's selected shipping provider (e.g. Fedex, UPS, Australia Post) during checkout. The configuration is possible on Discount edit page -> Requirements.
With this plugin it will be easy to configure discounted shipping rates for your customers.
For more details please visit this page. You may also find a plugin for your older nopCommerce version there!
For documentation please visit an official nopCommerce discount documentation here.