I don't see a way you are going to win this one.
You were parked in an area that was not a designated parking spot.
You ran in to go to the bathroom on top of that.
I see this all the time here at the supermarket here. Everyone thinks they are running in for just a few things, and can park in the firezone here. You really are not supposed to do that. It causes a hazard because they are blocking the firelane.
I know you know the stripped area next to the handicap spot was your indication that's not a place you can leave your car and go in and go to the restroom. Noone is going to be expecting you to be parked there because it was clearly marked as not a parking spot.
I'm really sorry, but I just don't see what you can do to defend that.
Around here, I see police pushing people out of the firezone while they wait for someone to run in for "just a second" all the time.
Unfortunately, you didn't get a warning, they gave you the ticket.