We are concerned that we dont understand your pricing for upgrades

 If we pay $19 per month in anticipation of 10 customers sending payments via the form, what happens if we gain more customers in that month?

We don't see anywhere that you indicate how we would know that more customers had attempted to use the form or how we are to pay for additional customers for that month.  So, we suspect there is some reason that kind of information is missing?  Can you explain for us please?

You will be notified when you have reached any of the limits on your account, once you have reached the limits you will need to wait until the next month (1st EST) to get your limits reset or you may also upgrade your account to increase your limits. 

You can also check your limits here: https://www.jotform.com/myaccount/usage 

Now, once you have reached the limit your forms will be temporary disabled and your customers will receive the message while they try to fill the form. 

If you have more questions, let us know.