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Yes, you can limit the quantity of the products that you would like to sell by using several widgets in the same time and I see that you are already on a good way to achieve this.
I would recommend checking this easy to follow tutorial here: How to pass a calculation to a payment field. It will show you how to connect the fields using conditions so that you can pass the price to PayPal.
I should mention that they will not see products on PayPal pages this way, they will only see the price.
Do let us know if you have any further questions or require an assistance with anything.