Our site requires users to pay a sign up fee, but we don't want the sign up fee to prevent users from creating an account on our site. Our plan is to allow the user to create an account without having to pay for the sign up fee. They would be able to access a demo version of the site, but they would have to pay the sign up fee later. But how to present that the user has to pay the sign up fee but pay it later and to remind the user to pay? Would we have something similar to Wikipedia's method for requesting donations, where the form for the registration fee is at the top of every page?