No, my comment was ambiguous, sorry.

Anyway, right now your code has three major parts: if there are params, it is processing form input and then displaying a receipt; otherwise it displays the form. The validation goes in the "processing form input" part. If any of your validations fail, you want to print an error message, skip displaying the receipt, and go on to display the form. I hope that helps, or at least gets you on to the next specific question.