I've shipped a few standard packages to Canada - they have a tendency to scan and track the normal customs number, bless 'em 😄 and as the majority of my packages are valued $30 or less, that's how I usually ship. It's annoying that the one package I'm having issues with is - according to Australia Post - reliable delivery in 7-10 days and fully trackable. Hmmph.

(Never got any results for the customs number on US packages when just a normal letter, though, but I take a scan or photo of the package with the customs form attached for all packages I post that way, just for my own records and in case it ever comes in handy).