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Your last form ID is http://www.jotform.us/form/50418659853162, is this correct? Basically, you wanted to add a textbox for submitters to input an amount as a tip? If yes, a workaround would be to add another item in the payment configuration, put 1 as the price then enable quantity box.
Here's how you can add a custom quantity box: http://www.jotform.com/help/225-How-To-Add-Options-In-The-Payment-Form (Part II in the guide).
I've cloned your form and made a changes to it based on this suggestion, kindly check:
You can hide the price of the tip item using a custom CSS codes.
Let me know your feedback.
In relation to your other thread about dividing product sections into sub-sections, I've tried posting a reply three times and for some reason the replies keep being tagged as private. As such, I've created a new thread with your question and my response. You can find it here: http://support.jotform.com/answers/527249