Order form

The link to my order form is

http://form.jotform.us/form/50447378400150

The customer will come to the page, select the number of posters they want printed and select the image files for upload and printing.

I need the form to calculate the total number of posters ordered and provide that information to PayPal so that paypal can generate an invoice and charge the customer for the number of poster print orders submitted.

Please advise.