JotForm

https://www.jotform.com/

You can add payment forms using PayPal and many other services. https://www.jotform.com/order-forms/

Get the full source code and insert it using the code section of the Embed function.

If you modify the form you must get the new source code and insert it again.

https://www.jotform.com/help/104-How-to-get-the-Full-Source-Code-of-your-Form

Embed instructions for Classic Sites

Another method is to create a Web App. To create Web App for this see these instructions and see my Web App code.