I'm glad my second lens was a 50mm. It lets me not be so close to people when I take a picture. A little too long for taking pictures inside the house.
What body were you using the 50mm on? For the price of the 50mm I am probably going to pick it up shortly, but putting it on the XTi it will work more like a 80mm.
There is no duty to begin with. Only a custom brokerage fee and sales tax depending on the province you live in. USPS is the safest way to go. Many items come thru without even attracting the sales tax. the couriers are the ones you have the most trouble with because the item always goes thru customs. So avoid them. If you are concerned about the sales tax get the sender to put a lower value on the item.
I would say 600 is too much for the 28-135 in used condition
Paul is correct. There is no duty on photographic equipment. Only GST+PST (or whatever your province tax may be). If it's marked (properly) as USED photographic equipment, you may pay even less.