I have a online store, and 50% of the small under $500 items I ship by USPS get held up in customs, sometimes for up to 8 weeks. Naturally, this has customers screaming and wanting refunds. Eventually they do get it, and see the postmark date was 4-8 weeks earlier.
On the other hand, UPS is quick and efficient, but for standard shipping, they charge a fee for passing it thru customs, and they add the taxes required by Canadian law. Naturally, this causes customers to be unhappy as well.
You can't win!
BTW, ship UPS or FED EX with premium express shipping, and the processing fee is included in the cost, but Canadian GST will still be collected.