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.""

might be agreeing to participate ethically with other european nations in the slave trade

or notifying other european nations when taking possession   of african territories

its main object was the establishment of "settlement houses" in poor urban areas, in which volunteer middle-class "settlement workers" would live, hoping to share knowledge and culture with, and alleviate the poverty of, their low-income neighbors.