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.
To my understanding, you want to sell something and used installment payment.
Yes, it is possible. You can use recurring payment option. Here's a guide: https://www.jotform.com/help/166-How-to-Setup-a-Recurring-Payment-Option. You can set the amount for installment payment and date when to be paid.
Hope that helps. Let us know if you have any other question. Thank you.