Constitution of National GST Appellate Tribunal sets tribunal structure, appointment criteria, powers, and post office practice restrictions. Establishes a National Goods and Services Tax Appellate Tribunal headed by a National President with State branches called State GST Tribunals led by State Presidents; each State Tribunal to include prescribed Judicial and Technical Members, with qualifications, selection, appointment, and presidential powers prescribed on GST Council recommendation, and a prohibition on former Presidents or Members appearing, acting or pleading before the Tribunal after leaving office.