Which document allowed the colonists to make treaties and establish a postal service

The Articles Of Confederation gave the united states the power to establish the U.S. postal service, as well as make treaties.

https://www.history.com/topics/early-us/articles-of-confederation

"But Congress would exercise considerable powers: it was given jurisdiction over foreign relations with the authority to make treaties and alliances; it could make war and peace, maintain an army and navy, coin money, establish a postal service, and manage Indian affairs; it could establish admiralty courts; and it ..."

https://www.monticello.org/site/research-and-collections/postal-service

"Benjamin Franklin was appointed as postmaster general. Despite a clause in the Articles of Confederation to establish a federal post office, Congress waited until October 18, 1782, to pass "An Ordinance for Regulating the Post-Office of the United States of America.""

on april 6, 1917, the u.s. joined its allies--britain, france, and russia--to fight in world war i. under the command of major general john j. pershing, more than 2 million u.s. soldiers fought on battlefields in france.