Postal workers or USPS employees are not allowed to carry mace or any pepper spray that they buy personally.
USPS though does issue USPS provided dog repellent spray but not mace or regular pepper spray.
If you're caught carrying mace or other pepper spray that is not issued by the USPS then you can get into trouble and be fired.
So don't carry a gun or your own mace because if caught you can get into trouble and also no guns can be carried onto postal property or mace or pepper spray.
But you can have the dog repellent that the USPS supplies you.