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JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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If I understand correctly, you want your submitter to complete the payment first, and then submit the form, is that correct?
I have checked your form id#51533358439157 and observed you are using the simple Paypal integration, my best advise for you is to change to Paypal Pro.
Please take a moment to read this helpful guide that explains the difference when using Paypal and Paypal Pro: http://www.jotform.com/help/183-Difference-Between-PayPal-PayPal-Pro.
I hope this information helps, if we can further assist you please let us know.
I am sorry that I was short in my answer above, as I have also observed that you have received two sobmissions in that form, can you please let us know if payments were also processed?