Registered Mail provides insurance for valuable first-class and priority matter. Registered mail must be sealed against tampering.
Do not use self-sealing envelopes.
Wrap and seal packages with mucilage or glue or with plain paper or cloth tape.
Do not seal packages containing currency or securities exclusively by use of paper strips. First seal such packages securely with mucilage or glue.
Do not use slick-faced tape, e.g. cellophane, masking, or nylon wrapping tape