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Basically this is closest workaround we have when it comes to a stock control(It is not applied directly to the payment integration):
1)Add the Inventory widget: http://widgets.jotform.com/widget/inventory
You would need to add each time this widget to create a single product:
2) Add the Calculation widget on your form:
Just to let you know, a total amount will be passed to Paypal side, there will not be description of the each product since it is not apply directly to the payment integration.