I want to create a simple form where customers can just pay for my services on line and, in turn, get a receipt via email or their payment.

Here are a few guides that should be related to what you are looking for.  This one is on how to set up order forms:

https://www.jotform.com/help/47-Order-Form-Basics/

The next couple are related to setting up emails notifications for your forms. 

The first is on setting up notifications sent to you:

http://www.jotform.com/help/25-Setting-Up-Email-Notifications

The second is how to setup notifications to your users:

http://www.jotform.com/help/26-Creating-a-Form-Autoresponder

If you have further questions, let us know and we will be happy to help.