How did bilateral postal treaties make international mail difficult in the 1800s?

In the 19th century, international mail was hindered by a proliferation of bilateral postal treaties that created a chaotic and inefficient system. By the 1860s, many European states were party to at least a dozen such agreements, which necessitated the maintenance of highly detailed and complex interoffice accounts between countries.1

These treaties made international mail difficult for several reasons:

• Administrative Complexity: Because there was no standardized system, postal administrations had to navigate a "bewildering variety" of different currencies, units of weight, and measurements. This made the accounting process incredibly cumbersome, described by a U.S. postmaster general as "almost beyond belief".1
• High Costs: The lack of a unified framework resulted in high international postage rates, which placed a significant burden on users of the postal service.1
• Lack of Uniformity: Unlike the domestic reforms introduced by Rowland Hill, which simplified procedures through uniform rates and prepayment, international mail remained fragmented until the establishment of the Universal Postal Union in 1874, which finally provided a uniform framework of rules for global mail exchange.1