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JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Simply use our Paypal Payment Form, it's on the first list of the Payment Tools. Anyway, I can see that your form is already redirecting to Paypal after selecting one of your product and hitting submit button. Though, the only problem here is that your product field isn't required, this makes it possible for the users to ignore the field. I would suggest to make it required instead.
To make the field required, simply click the payment field and then click Required option. Example: