Build plugin for Shopify

We develop sales tax API for Shopify. Shopify doesn't calculate tax rate for each state We have the Shopify integration with our software and we can receive data from Shopify. But we want to give the tax rate, we need to calculate this tax rate and give it to Shopify and after it the client can see this tax rate when he pay for the order. And for it we need to build a plugin that can provide a tax rate solution at checkout. We assume that this can help us [login to view URL]