In 1991, President George H. W. Bush signed Executive Order 12770, Metric Usage in Federal Government Programs directing all executive departments and federal agencies implement the use of the metric system. As a federal government agent, USPS should obey the law, and implement the metric system immediately throughout its postal offices.
It is ridiculous for the USPS to keep using an outdated english system when the english don't even use it any more.
You wonder why people in some countries hate usa, it is because USA thinks that it is special. What is so special about the english system when even people in the usa think it as a decimal system.
People change their cell phones almost yearly, so there is absolutely no reason why the UPS can't change its computer system which can be done without updating computers.