Hi,
I figured I should add a few things to this as I have a very close relative that works for Canada Post in a management position.
First the best way to get a good shipping cost from Canada Post is to get a contract, if you are willing to meet their required amount of parcels mailed per year.
They have a whole range of discounts based on the number of parcels you sign your contracted guarantee for.
All parcels need to be manifested though.
Next Airmail just gets flies to the nearest gateway city in the USA from there what the USPS does with it is up to them but normally it flies to the city it is intended for.
Surface mail gets trucked to the nearest gateway in the USA and again the USPS does handles it the way it feels best, normally due to their sheer volume it gets handle by plane but no guarantees... So the 1-2 day difference is caused mainly by the first leg of trucking the item across the border. Therefore unless your far from a parcel sorting plant, there really isn't much difference in the time delay between surface and airmail within the same zone... but when you have small parcels going from say westcoast of Canada to the eastcoast of the USA, Surface mail parcels mailed from say Vancouer will just be trucked to Seattle, enter the USPS from there and then the USPS will take over and either truck it or fly it depending on the volumes to go east. When you ship the same parcels airmail, it will probably fly from Vancouver to the Toronto and then across the border to the USA and the USPS will take over the handling of it.
So for me, I ship small packet surface for anything on the westcost of Canada or the USA surface mail, anything going east I will send Airmail.
We are currently assessing our volumes of parcels from last year and figuring out what type of increase in parcels were expecting and will probably sign a contract based on the information to get an even cheaper rate as well as the tracking numbers for the parcels.
If you feel you don't have enough parcels on your own for a contract maybe ask some other sellers in your area to join a group and put all your parcels together to come up with a number of parcels you could contract with Canada Post... Personally I don't find driving the 20 mins to cross the border worthwile to me, as numerous times crossing the border takes a lot longer with the lineups etc, and any returned items costs you double + for not shipping with Canada Post.