What language is your app written in? You can get the IP from your server. You can pass to auth.net and retrieve it later. You can also just capture it and store it in a db. What is your use case for this? If for security, I would capture it and log it at the time the pay button was clicked.
I am almost certain, back to your original question, that every server side programming language has a method to capture the IP address. In php it is $_SERVER[‘REMOTE_ADDR’]. That global variable is the users IP address.