If customers have some shippable products, they can choose a shipping option during checkout
These shipping options are returned from shipping rate computation methods (such as UPS, USPS, FedEx, etc). Shipping rate computation methods are implemented as plugins in nopCommerce.
We recommend you read How to write a nopCommerce plugin before you start coding a new shipping rate computation method.
The article will explain to you the required steps for creating a plugin.
Getting shipping options.
Now you need to create a class which implements IShippingRateComputationMethod interface.
This is the class that will be doing all the actual work. When nopCommerce calculates shipping totals or needs to get a list of available shipping options, the GetShippingOptions or GetFixedRate methods of your class will be called.
Here is how UPSComputationMethod class is defined (“UPS” method):
/// UPS computation method
public class UPSComputationMethod : BasePlugin, IShippingRateComputationMethod