The application describes a container, bag, pouch or reusable envelope which is closed by one part (2) locating over a second part (1), the second part(1) having an opening (7) for receiving the head (14) of a breakable seal (13) which locks into the opening in such a manner that it cannot be removed without breaking, the seal (13) in its inserted position locking the first part (2) in place over the second part (1) in a tamper evident fashion. The second part (1) has a shoulder (17) at either side of the opening (7) and the seal (13) has on its underside recesses (19) for registration with the shoulders (17). Typically said one part (2) is the fastener tab of the zipped opening of a reusable envelope and the second part (1) is the enclosure or chamber of a lock unit mounted on the envelope, adjacent one end of the zipped opening.