TAX PROVISIONS - HARMONIZATION OF LEGISLATION - TURNOVER TAXES - COMMON SYSTEM OF VALUE-ADDED TAX - EXEMPTIONS PROVIDED FOR BY THE SIXTH DIRECTIVE - EXEMPTION FOR THE SUPPLY OF SERVICES BY THE PUBLIC POSTAL SERVICES - EXTENSION TO THE SUPPLY OF SERVICES BY OTHER BODIES ON BEHALF OF THE PUBLIC POSTAL SERVICES - NOT PERMISSIBLE ( COUNCIL DIRECTIVE NO 77/388 , ART . 13 A ( 1 ) ( A ))