The US Postal Service posted following notice on the Federal Business Opportunities website:

A successful manufacturer of seals shall be capable of producing anti-pilferage seals with the minimum general requirements listed below:

The seal is intended for use on mail containers where security is of utmost importance. The seal shall secure the mail containers and shall not allow the container to be opened without physical destruction of the seal. The seal shall be a passive one time use locking device and shall indicate tampering or access into the mail containers. The seal shall close and lock in a manner that provides an audible and tactile feedback to the user that the seal has closed securely. When forcibly opened or cut, the seal shall be incapable of being resealed. The seal shall be protected against “picking”, i.e., to unlock and re-close the seal without evidence of tampering. Any attempt to open the seal should be readily detectable.

Each seal shall bear a printed barcode that encodes its unique sequential serial number. The barcode shall match an eight (8) digit identification number embossed on the seal. The barcode shall be compatible with USPS scanners and utilize one of the following barcode symbologies (to be specified in the future): GS1-128; ISS Code 128; DataMatrix; PDF-417; Interleaved 2-of -5 (I 2-of-5); or Code 39. At least 70 percent of the barcodes in each shipment of seals shall have an overall symbol grade of “B” or better. The remainder shall measure no less than a symbol grade of “C”. Specified symbol grades are based upon the ISO/IEC 15416 Barode Print Quality Guideline (formerly INCITS 182) which recommends a method of measuring the quality parameters of printed barcode symbols.

source: Federal Business Opportunities