Carly, I think eBay should formally go to bat for us sellers with Canada Post. You have the statistics and a letter from you opposing the rate hike will have more effect than a few from us here.
I'm personally investigating going with a courier company. We are shipping over 100 parcels a week now and our business is just growing. Canada Post has disgusted me enough...
If you are shipping that much, contact UPS and have them send someone in to give you a quote. I have cheaper rates with them than I would with Canada Post (to the U.S. anyway) and they have proven to be very reliable.