The shipper can in many cases specify that they are to be billed for customs and clearance, but I've never done it other for an item going to the USA. Unless you are a big bonded shipper with US accounts, you have to wait for US Customs to send the bill to Canada Post who then sends it to you, and then you pay CanadaPost who pays US Customs who releases the package to USPS for delivery.
Prior to 9/11, that would all take about 10 days, but now, US Customs will have every parcel lined up for inspection so don't expect the package to be on a USPS truck for at least 30 days, and that's the most optomistic scenario, as 6-8 weeks is closer to reality.
If you ship with a parcel service like UPS Fedex et, if you have an account in good standing, they will pay and bill you. Of course They get around $40.oo for clearing it where as Canada Post charges $8.00
It's a good way to do it so the customer is insulated from all the escoteric nonsense, but it's really only viable for a large distributor or manufacturer, at which point you are probably doing your own truckloads and use a commercial broker. For the same package that UPS charges $40, a commercial customs firm will charge me $18.oo, but that rate was prearranged and shows on my account.
eBay sellers are better off not pre arranging customs because that means the parcel must go through customs where as letting the buyer deal with it means it may or may not be subject to clearance.
The US does not have taxes, the monies a buyer might pay are related to clearing the shipment unless you are selling one of the rare items that still has a duty applied to it, which is mostly agricultural prodcts.