Most web form builders offer online payments via PayPal integration. But if you look at the pricing page carefully, you'll see that this option is only available in top plans or you have to pay 1% from each transaction. Not cool.
But don't despair, if you are looking for free web forms that are integrated with online PayPal payments - Bitrix24 does just that at no cost to you whatsoever. There are a number of advantages of offering online payments inside web forms. Suppose you are a consultant that runs workshops. By giving your clients opportunity to sign up and pay for the seat right away, you are saving yourself a ton of time. You don't need to worry if the person had paid for the workshop or not.
Better yet, because Bitrix24 web forms are integrated with the CRM, you can insert product catalog inside the form. So if the person wants to pay for the workshop and 10 hrs of consulting, the form will tally up the total and combine the payments into one.
Bitrix24 CRM manual is available here.
If you are interested in buying self hosted editions of Bitrix24, which come with API, CMS and source code, and can be easily customized
and integrated with other tools, please download trial version here.