JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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.
You can set your payment field as required so that form users will be compelled to select a product and make purchase on your form.
To do this:
1) Edit your form and select your payment field
2) Click the Required option on top
Once the Required option is enabled, save your form. After which, from here moment on, your form should not submit without any selected product in it.
If you have questions, please let us know.