It would be a lot more elegant to write it in MVC, with a Controller class loading the page with all the forms and default or previously inserted values and, when required, calling a model method to insert into database and possibly send a pre-formatted email. That way spaghetti code and redundant code can be avoided. That's my first thought. The second thought is that it could possibly increase the loading time, but I don't think that's something noticeable to human. But I haven't measured it, I'll definitely try and see what the difference is.
Had I planned this ahead I would have written in MVC, but I decided to add the server side on a whim, that's why I added both index.php and index.html to my submission.