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 am not quite sure if your payment gateway allows for your requirement. However, it is indeed possible to send the post data to an external URL using webhooks. Here's a guide on How to Setup a Webhook with JotForm.
You can use the Jotform API to send a notification. Here's an example: Webhook PHP Script Examples
Here are several use cases for the Jotform API that you can check out: http://api.jotform.com/docs/#usecases
If you have further questions regarding the API, you can also email our API team at firstname.lastname@example.org so they can properly address your requirement