I ship to/from Canada and US about once a week. USPS Priority, Express, and First-Class are all turned to CanadaPost. USPS Global Express Guarantee (GXG) is turned over to FedEx. USPS Priority takes 7-10 business days whereas USPS Express takes 3-5 business days. GXG takes 1-3 business days.
Here are some tips about customs, taxes &duties:
USPS Priority and Express - declare value under $100 and check the "gift" box. No customs or duties upon delivery! If it's over $100 and/or the "gift" box is not checked, you pay $5 for USPS Priority ($10 on USPS Express) on top of GST+PST. These are flat-rates and are often considered "customs clearing charges." The amounts may have gone up recently so I'm not too sure on the actual dollar amount of these flat-rates.
For FedEx and UPS, unless all taxes and duties are paid for when the sender ships the package, you will be required to pay a hefty amount upon delivery. FedEx, UPS, and DHL charge "customs clearing charge" of $30 on all packages, even if the actual value of the item is below $30 and a gift. Sometimes, businesses make you pay taxes and duties at the time of the purchase. The same rules apply, but you pay these fees at the time of check out, not at the time of delivery.
Also, USPS does not use a "tracking" number. They use a "delivery confirmation" number, which as the name implies, just proves the item has been delivered. On USPS.com or Canadapost.ca , you can enter in this "delivery confirmation" number and get some updates as if it was a tracking number but not always. Also, of the 100+ packages I've sent and received, only one package sat at customs for a week for no reason.
Also, another thing that is nice about USPS/CanadaPost, If you miss a parcel or letter, it gets delivered to the closest post office. With UPS, DHL, and FedEx, it gets sent back to the hub, which is usually out in the boonies so it'll take a little while to get there. Not to mention, you have to call the 1-800 number before 5-6pm to get a "will-call" placed on the package so they can prepare it for pick-up that night (usually have a 2-3 hour gap to pick it up that night, if not the next day).
Hope this has helped! If you have any more questions or need clarification, please do ask!