Is there any way to restrict submitting a form that is using the Purchaser Order gateway if the order total is not zero?

I have a form that we are using for an award giveaway data collection point. Each end user will be receiving a coupon code according to their earned award. We currently don't want any credit card options, but we don't want the form to be submitted if the client's coupon can't cover the total charges. I can't figure out how to make that work. Please take a look at our form and let me know if there is any way to do this.

This is possible to get a total value using a Form Calculation widget field.

Then, setup a Hide field condition, if the value is greater than 0, then hide the form submit button.

If you need any further assistance, please let us know.