We use a number of other nuget packages and have found that there is a collision with the configuration settings used in app settings. In this package, you've got the following two keys set in the web.config.transform file:

<add key="base_uri" value="https://api.createsend.com/api/v3.1" />
<add key="base_oauth_uri" value="https://api.createsend.com/oauth" />

Can these be changed to something more specific, say, createsend_api_base_uri and createsend_api_base_oauth_uri?