Joint Custody Agreement
In legal terms, a contract is any agreement between parties to exchange things of value, such as goods and services for cash. A simple handshake agreement is legally binding, but its terms are not always provable if a dispute arises. However, you don't necessarily need pages upon pages of legalese. A basic contract agreement often works best for a simple business transaction.
Conventions and Treaties signed by signatories of states bind private actors to the terms of that agreement through the domestic application of the treaty provisions. These agreements may be bilateral (between two countries/states) or multilateral (between three or more countries/states).