What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Your form is integrated properly with PayPal. I did a test submission and it redirected me to the PayPal page. Here's a screenshot:
I checked your submission and it seems that the user did not "check" the product page. If this is un-check, then it will be treated as a regular submission and not a payment one.
I suggest you to make the payment field "required". In this way, your users cannot leave it blank. In the Form Builder V4, select the payment field and then click the "Properties" icon. After that, under the General tab, you'll see the "Required" switch. Please turn it ON.
Hope that helps. Thank you.