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.
I checked your form and I was able to choose a product. Here's a screenshot:
If your payment field is using check boxes then it is possible that the product is set to "Required". If the product is set to require, then you won't be able to click the check box.
Let us know if you're still having issues with your payment field. Thank you.