I'm working with an enrollment form for a prepaid card right now. In certain situations (when the user doesn't provide a social security number) the user will need to send a copy of their ID to the company before they can receive their card. In many cases the user will not do that. My theory is that they do not read through the final page and therefore don't even know that they are supposed to send their ID. I guess this is a natural thing, because usually when you submit a form and the next page does not have further form fields, you assume that you are done.
One idea I had was to send a few (2-3 in intervals) reminder emails to the user. But I also thought that there must be a way of getting the user's attention right after they complete the signup form. I was thinking about an in-page popup (like a light box) with a short warning, bur maybe there are better ways to do this?