Am I Allowed to Use Tape to Close a Postal Envelope?
Tape can be used to close a postal envelope or a package. Tapes that are acceptable for both closure and reinforcing envelopes and packages include pressure-sensitive and gummed tapes. These can be plastic, cloth or paper tapes.
The United States Postal Service says that the tape must be used properly when applied for the best closure. A gummed tape requires adequate moistening. The surface must be clean and at an above-freezing temperature since cold can cause the tape to crack. Pressure-sensitive tape can be applied to cold surfaces but needs to be rubbed down after application.
The common household tapes, cellophane and masking tapes, cannot be used to close an envelope. These tapes can only be used as reinforcement of another closure, such as a self-adhesive envelope.