It sounds like you're using CanPost's regular rates.
If so, register for CanadaPost Small Business which gives you access to other shipping choices not available for "regular retail" customers and lets you create a shipment & pay on line. All you have to do is print out the shipping label, attach to the package & drop off @ post office.
Rates to the US depend on size & weight of the package — CanadaPost uses "Volumetric Equivalent" so a box 30cm X 30 X 30 with a 2kg weight will get charged as though it weighs 7kg. The trick is to use the smallest box possible that still provides adequate protection for the contents.
If your item fits "Small Packet" requirements for weight & box size and isn't worth more than $100CAD Small Packet is often cheaper than sending the same package to a Canadian address. Go figure!!! But that's CanPost...
I just bought an item on eBay out of the US: shipping was $28CAD, I could have sent the same item to the seller for $12.
Maybe point out to your buyers that they're also getting around a 30% break with the exchange rate.
