It was actually started by the NALC (National Association of Letter Carriers)

The USPS wouldn't be involved if they weren't going to get a ton of bad pub by disallowing it. It costs overtime, ya know........

The letter carriers and Rural carriers combined with volunteers do the work. Management has actually mandated in some offices that the office not participate.

Just posting this to give credit where credit is due, it's those pesky Unions, not the USPS behind the drive.