What type of postage must an anonymous mailpiece bear?

An anonymous mailpiece must bear stamps only because this form of postage allows the sender's identity to remain concealed. Stamps are physically affixed to the mailpiece, and since they do not carry any identifying information like a return address or sender's details, they ensure anonymity. This is particularly significant for individuals who wish to send a mailpiece without revealing their identity or contact information.

Other forms of postage, such as metered postage, custom printed labels, and bulk postage, typically include identifiable information that can be traced back to the sender or the mailing process. This traceability goes against the purpose of anonymous mailing, where the sender intentionally does not want to be known. Therefore, stamps are the only postage type uniquely suited to maintain the sender's anonymity while also ensuring that the mailpiece is adequately paid for.