diplomatic bag 
noun British
A container in which official mail is sent to or from an embassy, which is not subject to customs inspection.Kissinger used diplomatic bags to smuggle lethal weapons past the customs of a democracy; he is potentially outside the law as much as any terrorist....
- What do you think is in all those diplomatic bags that they carry all over the world?
- Under international convention, diplomatic bags are supposed to go through customs unimpeded, and the European Union has also protested against Zimbabwe's action.