FWIW, I sent a label maker to Canada and had to pay customs charges. Went USPS.
This strikes me as very bizarre and I can't work out how that would be the case.
Canadian Customs Charges are the responsibility of the Canadian recipient, not the US sender.
When USPS sends a package to Canada, the first recipient is CanadaPost.
CanadaPost then turns over international packages to CBSA (Canada Border Services Agency) who then make the decision whether or not to levy Customs Charges.
Customs Charges and sales taxes can be waived for small value shipments. Often the charges consist only of the Federal GST plus the Provincial PST taxes plus a handling fee of $9.95.
After their assessment of the package and any Customs liability, CBSA returns the package to CanadaPost for delivery and fee collection.
If the carrier is UPS, they will, at the destination, offer to act as the recipient's Customs broker, a service for which USPS charges a significantly greater fee than stated in the last paragraphs.
I have received a large number of packages from the US via USPS, which I much prefer over UPS or Fedex or any other private courier.
I would highly recommend USPS, with later handling by CanadaPost, over any private courier.
Can't speak for AustraliaPost at this time, but having lived there for 10 years in the 70s, my recollection is more expensive Customs/Postal processes at that time than what I experience in Canada.
My main point is that I would encourage the use of USPS. I have never had any problems of any kind over the last 10 years.
As for tracking, CanadaPost coordinates extremely well with USPS tracking ... amazingly well. I would think that AustraliaPost would also.