100% off Coupon Code

https://pircheidaycamp.org/2016/registration/
I have created a form with payments set up through paypal.
I am trying to figure out 1 of two things.
Ideally I would like to give people the option to pay with postdated checks and therefore fill out the form to submit their registration information and then instead of paying the full price through paypal, I will only charge them $10 x the number of people they are registering (as a registration fee). I am not sure how to do this using a coupon code. I want to create a code that will work for everyone and not have to make a different one per user.

If there is no way to do that my second option would be to 1st have them fill out a request postdated checks form https://pircheidaycamp.org/2016/scholarship/
and charge them the $10 registration fee, and once I have approved their request I can send them a 100% off coupon code to go back to the original form, fill out the information and then having already paid the registration, they will send in postdated checks for the rest that they owe. However when I try to create a 100% off coupon, when I click pay now, the form does get submitted, but the user is sent to a “this page cannot be found page” How can I create a page that will say, we have received your registration, please send in your postdated checks as we have discussed by June 25th.

Thank you.

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