I own and operate a UPS Store in Nor Cal and Hacksaw is right on target with regards to UPS and USPS. Low cost items shipped UPS using Canada Standard are pretty much the same as USPS rates, but you do get the benefit of tracking. Higher priced items do seem to pass through customs more smoothly with USPS - and many Canadians request this service as customs fees don't seem to be as harsh - or maybe they are overlooked?
Lower value private party sales that can be processed as "gifts/personal items" won't face the 'taxed & tariffed' scrutiny that items sold through retail stores face.
Recently I sold an item on Ebay which had several bidders - almost all of them were from Canada - I am sure this is directly related to the poor performance of the US dollar. Ulitmately the winning bidder was from Canada and the item was shipped UPS. There were several delays caused by customs and the buyer had to pay additional tariffs before the item was delivered.
Items shipped Parcel Post via the USPS are a low priority for them. These packages can sit in a warehouse (hub) for a week or more until there is room on a truck for them. If your package has to pass through two or three warehouses, this could add two or three weeks to delivery - just to get your item to the Canadian border. The date range provided by USPS is only an estimate based opon best case routing. They do not actually provide guaranteed time-in-transit delivery. A cheap option (under a buck usually) is to ask for delivery confirmation. This creates an element of tracking within the USPS system. While you might not get the online tracking to check yourself, the Postmaster can ususally trace them item by the delivery confirmation number.
Usually smaller boxes, under say 5 pounds, can go USPS Priority International to Canada for about the same cost as Parcel Post - and you should receive the item much quicker.
I hope this helps a little.
Dennis
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While I have long believed that I will never get old, I have come to the realization that sooner or later there will be more people younger than me.