Your method of finding the list number in Constant Contact is now outdated. They no longer show that information.

Here’s the post that explains it: https://tinyurl.com/l4harap

Instead, once you have your API key, use the interface to run a test to get your list numbers.

Here’s a link to a screenshot: https://imgur.com/XHRDqlL

https://www.ads-software.com/plugins/shopp-constant-contact/