Shiptheory integrates with eBay. Shiptheory will pick up any shipments from orders that Shipped. Shiptheory will process these shipments through the Shipping Rules you've created in Shiptheory.
This article explains how to Config eBay and Shiptheory.
Make sure you're logged into Shiptheory, then navigate to https://shiptheory.com/integrations
Find the eBaybay integration in the list and select enable:
Make sure Item Shipped box is ticked, then hit the connect button. This will redirect you to your ebay login. Login as normal, and you will be redirected to Shiptheory.
You will notice that you now have your eBay Seller ID. This is just for a record, it is not used and cannot be changed here.
You should see the 'Your eBay store is connected' message in green.
If this is the case, you're ready to start marking things as shipped in eBay, which will send orders through to Shiptheory.
You may want to look for more information on creating your shipping rules, see here.