How can I set inventory control limits with PayPal?

We are selling "tickets" to training events.  In the past, we used the inventory control features within PayPal to set the maximum number of "seats" we sold to an event.  This prevented people from registering and paying when there were no available spots.  We have a new registration form created in JotForm, and we are using the PayPal button as the mechanism for people to pay and then submit their registration.  How can we set the same inventory control features/limits in this form?  We want to be able to limit the number of submissions so that people do not submit a registration once all the seats have been taken.  I do not have the form posted/visible to our website yet, as we do not want to open registration until we have the inventory control features in place.  

For selling a limited number of tickets through your form, we recommend using a widget such as our Inventory widget, Gift Registry, Quantity Gift Registry, or any other similar widget:

https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form

In order to charge your customers the correct amount, you could base a calculation on the number of tickets they have selected in the Inventory widget:

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

So if 1 ticket is $10, and customer selects three tickets in the Inventory widget, we can calculate and charge them $30, while 3 tickets get deducted from the total available pool of tickets.

You would pass that calculated amount to your payment field for payment processing:

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

If on the other hand you will only have 1 ticket available per each customer, so that the number of times your form would be submitted is exactly the same as the number of tickets you have available, we could set this up in an even easier way by simply setting up a submission limit on your form:

https://www.jotform.com/help/137-How-to-limit-submissions-in-a-form

Please take a look whether these solutions meet your needs, and let us know should you need further assistance. Thank you.