You do not have to pay if your app does not make revenue.
Release and Royalty Tracking Guidelines - Unreal Engine Here is all the information you need. A step by step guideline.
Once you send the information. Epic will provide payment instructions.
Most probable that you have to send it them by yourself. A system that detects your revenue transered to your bank account made by your ue4 game is not present. But you are due to have a legit administration. (Like any other organisation)