I am in Canada, and have been doing this for years. The problem is, our Canadian Customs tacks on 15% of whatever is declared as the value of the item, not to mention brokerage charges and any applicable duties. This seems ridiculous to me, as we are supposedly in a 'free-trade' society in North America. I have shipped many items to the USA, and they don't seem to have any of these same importing Customs fees we do.
The way to get around this is ask the U.S. seller to declare a minimum value and fill out the Customs form declaring the item as a 'gift' or 'sample'. Canada Customs NEVER questions the amount, and it will sail through with minimal charges. HOWEVER, keep in mind that with this strategy there lies a risk. If the item is for whatever reason lost or damaged, you don't have much recourse to get anything back for it due to the low declared value.
I tend to take the risk, and instead put the extra $$ into a quicker shipping method (i.e. Express/Air), meaning there are less depots/stations and hands touching the item, hence a higher probability of it getting to me quickly and not getting lost. This strategy has saved me a lot of $$ over the years on Customs charges, and I have been lucky nothing has been lost (yet). Even cheaper is USPS Express. It is a veyr good service, and the brokerage charges are always less than UPS/Fedex.