score:4
Non-refundable does not mean non-changeable. If there is a cancellation fee or a change fee stated for the ticket, that usually means you have a changeable ticket.
In your case canceling the booking should result in the airfare you paid minus the cancellation fee becoming a credit that you can use for a future ticket on that airline (often with a time limit for use).
I don't think justfly.com can charge an additional fee on top of the airfare you paid.