You'll need your Amazon Seller ID and auth token when adding an Amazon store, or when adding Channel Advisor and using Amazon.
Step by Step: Obtaining the Amazon Seller ID and Auth Token
1. Click the link to https://developer.amazonservices.com and then click the Sign up for MWS button.
2. Sign in to your Amazon MWS seller account.
3. Select "I want to use an application to access my Amazon seller account with MWS. Then enter "ShipWorks" and our developer account number "1025-5115-6476" in the fields provided. Click Next.
4. Check the boxes accepting the agreements, then click Next.
5. Copy the Seller ID, Marketplace ID, and MWS Auth Token from the next screen. IMPORTANT: These are not automatically saved for you or emailed to you. You must copy these to a safe place, or print the page for future reference.