Payment by invoice

is it possible to pay for an upgrade by invoice? 

Yes, it's possible. We have the following invoice method:

https://www.jotform.com/help/353-How-to-Upgrade-and-Pay-by-Purchase-Order

https://www.jotform.com/help/352-How-to-Upgrade-and-Pay-by-Mail-Check-Money-Order

https://www.jotform.com/help/354-How-to-Upgrade-and-Pay-by-Wire-Transfer