After you registered a new application for Ecwid, send your shipping URL (your server endpoint) to Ecwid team. Ecwid will be sending order details requests to that endpoint and expect shipping rates in a specific format in response.
Next, start working on shipping calculations – Shipping request and response
If your application is for a public use, the request URL must be working via HTTPS. Also, the certificate can only be from trusted CA's and not self-signed.
Your application can require merchants to specify their shipping account details, package size and any other user preferences you may require.
Settings
Set up a new tab in Ecwid Control Panel, which will serve as a settings page for your users. This tab will load a page from your server in an iframe in a separate tab of Ecwid Control Panel. See Native Applications
When merchant is in the settings tab of your app, your code can create and modify the merchant settings using the Application storage feature. It's a simple key:value storage, which can serve you as an app database. For your convenience, you can access it via Javascript (client-side) or Ecwid REST API (server-side).
Request
When customer is at checkout, Ecwid will send app storage data in a POST request to your application URL alongside order details. The request will contain all data from your application storage, including public and other keys that were specified. Use it to identify the store and a user for the shipping rates.
Response
After you get a request from Ecwid, your application endpoint should get its components and return correct shipping rates back to the customer in a response.