How to pass the calculation value to Square payment?

I've been testing out a few different ways to allow students to purchase and register for my calligraphy classes using JotForm. I tried connecting it to paypal but that was unreliable and kicked people out or wouldn't go through to the payment section. I use Square for my in person sales so it'd be great to use it here too. However, there is no way I can find to limit the amount of a certain item that can be purchased. I can only sell 12 seats per class. For the paypal method I would use the inventory widget and form calculation to create a use defined amount. When I try to do the same thing with Square an error message pops up after hitting the submit button on the form. It does then go through but I can only imagine the panic emails I will get from customers about seeing that error.

Any advice?

Basically I need to accept payment for class spots and limit it to 12 spots for sale. Sounds simple, but it's gotten complicated.

You may choose User defined amount in the Payment type field and then pass the calculation value to Square Payment by selecting the calculation field in 'Get Price From'. Please see the screenshot below:

Hope this information helps! 

Yeah, that's how I do it now. When it's set up that way it gives the error message and then processes. But I need it to not show the error message. :)

What is the error message that is shown?  The payments should be processed the same as if it were products set up in the payment field rather than a value from a calculation field.  There should be no difference.