If the item is mfg'd in North America the buyer won't have to pay duties anyway.
Actually, only if it's originaly manufactured in Canada(Bryston, Classe, YBA, etc) would a Canadian buyer not have to pay duty on it.
I don't know what the weight restriction is for USPS, but I've had a 40lb cd player(50lbs total) shipped up here thru USPS with no trouble whatsoever.
Incidently, if you do(as an American seller) have duties charges redirected back to you, be sure to inform the buyer before assuming they're trying to avoid paying them.
I had an amp shipped from a reputable seller in ohio and thought I had covered all costs when it arrived.
However, it was only when I contacted the buyer months later did he inform me of the charges.
I was stunned!
Naturaly I reimbursed him(and purchased the second amp).