Five hours is a gross underestimate for the time it takes to build a robust and scalable webhooks delivery system, and even that is just for the delivery system

Five hours is a gross underestimate for the time it takes to build a robust and scalable webhooks delivery system, and even that is just for the delivery system itself. You also need to build the UI for your users, libraries for verifying signatures and etc. All of this assumes you know what you are doing and can just write the code. It doesn't even take into account the research you need to do to make sure you cover all of the security (and other) consideration that GP seemed to have already known about and took care of.