I have a Heroku app making some API calls to an external service. There are some issues and the service wants to know what IPs are making the calls.
A lot of the basic
According to the docs, Heroku Dynos can be any IP from an AWS Region. Having a dedicated/static IP for your Dynos is part of their enterprise offering.
However, you can have a dedicated/static IP by adding an add-on (e.g. Fixie or Proximo)
Also, you can have the Dynos make a GET request to https://httpbin.org/ip and it will respond with the Dynos public IP (more ideas in the comments section below).