I use Purolator for just about everything. I ship all my eBay sales, bill deliveries to me with my Purolator account number and ship local sales via Purolator. They will quote you based on estimated sales volume. 50 packages a month or 10,000 doesn't matter. They base your rate on an approximate dollar value.
They are very flexible however and will try to match or beat the competition. Depending on your needs, they will focus your discount rate based on your use. If you ship in envelopes a lot you will get a decent envelope rate. If you buy product by the pallet you will get discounts on your freight costs. If you primarily ship overseas... well you see where I am going. My opinion is highly biased but that is only because my local rep has been amazing with me.
When using their shipping estimator on the web site you can only enter by weight. If you actually went in to ship something you can enter in specific dimensions.
Purolator and I have recently created an "eBay seller" group account. In a nutshell, Purolator has agreed to pool my (and any seller interested) shipping volumes and give the large discount rate to everyone. Each seller would get their own account and pay their own fees but we share the discounted volume shipping rate. The best way to compare it would be to imagine the savings a nation-wide retailer would get due to their corporate account. I personally know a well-known electronics chain in Canada that can ship a 27" plasma TV to anywhere in Canada for about $10.
While our discount rate isn't there yet, the more people willing to give it a try will make it better for everyone.