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.
Thank you for contacting us.
If I understood you correctly you want to allow your users to enter custom quantity for product they select.
You can achieve that by checking Custom Quantity (Text Box) options, when you are adding new options to your products.
Also inside of this guide you can find how to add options to Payment Forms:
Here is my demo form: https://form.jotform.com/53424771319962?
Hope this will help. Let us know if you need further assistance.