Brexit: the "exit" part takes place over two years, but is irreversible once the process begins. Re-joining the EU requires unanimous approval from all then-current members of the EU.
Postal treaty: the exit takes place over one year, and is reversible at any point during the year. Re-joining the treaty is a simple matter of re-ratifying it through your legislature (i.e., the US Congress) because all UN members are automatically entitled to join the treaty and are not subject to any sort of membership vote. (Non-UN members are subject to a vote by the membership.)
Brexit: the "exit" part takes place over two years, but is irreversible once the process begins. Re-joining the EU requires unanimous approval from all then-current members of the EU.
Postal treaty: the exit takes place over one year, and is reversible at any point during the year. Re-joining the treaty is a simple matter of re-ratifying it through your legislature (i.e., the US Congress) because all UN members are automatically entitled to join the treaty and are not subject to any sort of membership vote. (Non-UN members are subject to a vote by the membership.)