Developing a plugin

I want to build a plugin for a different shipping software solutions provider (https://www.easypost.com/). What is the best approach?

Copy/Look at the source code of existing shipping provider plugins (e.g. US, FedEx, USPS).
But note that a "shipping provider plugin" just returns shipping methods / rates and some will support tracking.  They don't support other features like address verification, purchasing of address labels, etc.  There are some 3rd party plugins that have implemented those features, but they probably don't include the source code.