Editing forms that take payments

We're trying to build a form for a convention that has registration for the convention, plus more events. We would like to keep this simple and have only one submission per person. Is there a way to set up the form, using the PayPal payment option, that if they decide to add additional tickets to an event they can, but will only be charged for the new tickets? We don't want the people being charged twice for a registration fee.

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Example:

Person 1 registers for the convention and one dinner event. Total = $30 + 10 = $40

        They pay $40 through PayPal.

Person 1 then decides they want their children at the dinner, so they need to add two tickets. New unpaid total = $20.

         Total for convention is $60, but they already paid $40. PayPal only charges them $20 for added tickets.

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Is this possible???

Payments can only be accepted when the form is initially submitted.  Editing submissions will not trigger payments.  For any new tickets that need to be added, users would need to submit the form once again.