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 try following the method described on this user guide
Here is the demo jotform http://www.jotform.com/form/12972052359 base on the user guide.
As you can see there are multiple option on how to have payment on the form.
Hope this help. Inform us if you need further assistance.
In addition to what @jonathan suggested, you can also take a look at this thread to see if it better suits your needs. It allows you to set up different payment methods such as Paypal, Paypal Express and Paypal Pro, just to name a few, and it uses a conditional statement that allows the user to select a single payment method.