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After checking that form's Submissions page, I can see that there are 17 Incomplete Payments.
Are you referring to one of these incomplete submissions? If so, please note that submissions stay marked as Incomplete Payments until PayPal notifies us that the payment has been processed. In order to have PayPal notify us of this, you must have Instant Payment Notifications (IPN) enabled on your PayPal account. (How to Enable IPN for Paypal)
You can read more about it in our Paypal Incomplete Payments FAQ.
If you are instead having problems with a value of zero being calculated as price to be paid, I'm unable to reproduce that kind of a problem with your form. It appears to be calculating correctly on my end.
Is the form embedded on your website by any chance? If it is, could you please provide us with a link to the page where the form is embedded? This will enable us to check if there are any scripting conflicts between the form and the website that might have caused the calculation to fail.
Otherwise, your form looks fine when accessed over its direct link. We will await your response. Thank you.