Hi, I have worked with Gravity Forms a lot - have created addons for Confirmarions and Notifications, manipulated with entries data etc. I have also worked with various REST APIs, last one was Clickbank API I worked with several days ago. So in general I have completed dozens and dozens GF and API related projects and GF is my preferable forms plugin. I can code such integration plugin for $250 in 3 days. Some general info about me - I am wordpress plugin senior developer, just check my reviews they speak for themselves - I have 165 reviews and they ALL are 5/5, meaning all my employers were fully satisfied. :) I have successfully completed hundreds of different WP projects starting from minor theme updates and existing plugin modifications, to huge and complex WP plugin development and dozens of external API integration tasks. My hourly rate is $30, I am located in Latvia, North Europe, I am not a company, solo WP developer with 10 years experience so you will be dealing with me directly. Thanks, Dmitry.