What international postage agreement requires national carriers to use merely other national carriers?

Question

Whenever I do not care about shipping speed or times, I use national carriers to save money. Can anyone source or reference the quotation below? Which "international postage agreements"?

Private courtiers (DHL, UPS, etc) will always charge the duty because they collect a big fat brokerage fee from you. That's were you see the $50 charge on a $100 package.

Canada Post only charges a flat $9.99 on brokerage no matter the value of the package so I prefer to ship via national carriers (Royal Mail, USPS, Japan Post, etc) which becomes a Canada Post package due to international postage agreements [my emphasis].

Answer 1

Established by the Treaty of Bern of 1874, the Universal Postal Union - Wikipedia is the bases of the international postage agreements meantioned in your quote.