I'm not too sure on particulars of local mail transit within the region, but i hopefully can offer some insight on the international aspects.

In 1867 Transylvania became part of the Austro-Hungarian empire (as Transleithania). The Austro-Hungarian empire, the United kingdom and much of Europe were signatories of the Treaty of Bern in 1874, which established the "General postal union" (which became the "universal postal union" four years later, the name it goes by to this day).

The Treaty of Bern established common standards for international post between the signatories. Prior to this, international post was reliant on treaties between individual nations/empires.

The Treaty established rules covering pricing, stamps and that international mail be treated no differently than local mail.