The basic objective of the union was stated in the 1874 Convention, reiterated in all successive revisions, and embodied in the constitution: "The countries adopting this Constitution comprise, under the title of the Universal Postal Union, a single postal territory for the reciprocal exchange of letter-post items." The 1924 congress added: "It is also the object of the Postal Union to secure the organization and improvement of the various international postal services." The 1947 congress added another clause: "and to promote the development of international collaboration in this sphere."
In recognition of the union's continued interest and newly assumed responsibilities in the field of development aid, the congress held in Vienna in 1964 enlarged the UPU's goals to include the provision of postal technical assistance to member states. Under the single-territory principle, all the union's member countries are bound by the constitution and convention to observe certain fundamental rules pertaining to ordinary mail. Ordinary mail under the Lausanne Convention includes letters, postcards, printed papers, small packets, and literature for the blind, such as books in Braille. Although the convention lays down basic postage rates for ordinary mail sent to addresses in UPU territory, variations are permitted within generous limits. Postal authorities of all member states are pledged to handle all mail with equal care, regardless of its origin and destination, and to expedite mail originating in other UPU countries on a level comparable to the best means of conveyance used for their own mail.
In the past, foreign mail was delivered to its destination without charge to the country where it was posted, and each country retained the postage collected on international mail. Since mid-1971, however, where there is an imbalance between mail sent and received, the postal administration of the country receiving the larger quantity is authorized to ask for repayment at a standard rate (fixed by the Postal Congress) to offset its excess costs. However, each country reimburses, at standard rates fixed by the Universal Postal Congress, all intermediary countries through which its mail passes in transit.
Freedom of transit—the basic principle of the union—is guaranteed throughout UPU territory. Specific regulations provide for the dispatch of mail and for the return of undeliverable mail to the sender. Certain articles, such as opium and other drugs and inflammable or explosive agents, are excluded from the international mails.
Four optional postal agreements supplement the convention. They cover parcel mail, money orders, giro (postal checks), and cash on delivery.