So here's the funny thing...

I'd thought it would be nice and convenient to order new plate stickers online, except my plates expired in March and I still haven't received the new tags.

Yesterday, I received a $40 parking ticket because I parked on the road (paid parking) with expired tags.

Do I have a case to get the ticket thrown out?

I would say yes, you have a defence. You should have gotten an extended validation receipt when you purchased the stickers online. Your plates were valid, but unfortunately without the sticker the bylaw officer wouldn't know that. I'd try going to the local bylaw office and showing them the receipt (which should be dated before the ticket). I would hope they'd have a way of just cancelling the ticket without you having to go to Court.

Bylaws officers should really give a one month grace period with that charge since they can't verify if a plate's been renewed online. What a waste of your time.

This is one of the problems with the AMP system, no crown attorney, no JP, no trial: http://www.ontariohighwaytrafficact.com/post34547.html

Good luck OP, let us know the outcome.