It works good on bigger stuff, First off it cost a lot to ship bulky or heavy parts (less to the border), and weight/size limits are smaller on shipments crossing the border.
Works good if you are fairly close, we are 1.5 ish hrs from the border, takes a whole morning to make the trip..
On most smaller stuff, USPS works quite well (as Seaweb mentioned), if they do charge duty it is a 5 $ broker charge vs around 40? for UPS. I believe the UPS broker charge is the same on larger items, so 40$ is not so bad on 700$ value, but it sure makes you walk funny on a 15$ piece..
Shipping got way outa hand the last 10 yrs.
Also make sure that seller puts actual value on it, you get charged taxes, on declared value, and I think shipping cost.So if they fluff up the value, you pay more tax..