Tip Calculator App

So it's been a while I worked on any project. This project took me longer than I thought it should have but I'm glad at how it came out. My Javascript code is not really organized so please do well to comment on how I could make the code a little bit cleaner and more readeable. I had quite a challenge connecting up the error messages as well as making sure the custom field was in sync but I was'nt able to do this entirely right. On feedback on how I could fix this is highly appreciated. Thanks for checking out my solution :).