Although all delivery services serve a single purpose, they aren’t all the same. When documents or other forms of packages involve federal government agencies and some state agencies, you’re warned not to use private delivery services. The reasoning behind this is that private delivery services like UPS, FedEx, or DHL can’t deliver to P.O. boxes. Only the United States Postal Services can deliver to P.O. boxes and this singles out the private delivery services.
To put it in a simple perspective, since only the USPS can deliver to P.O. boxes, any other delivery service is a private delivery service. You’ll see this term used a lot on the Internal Revenue Service website where the agency cautiously warns taxpayers not to send their documents via a private delivery service. While it isn’t the case for every document and intended purpose, it’s how the general rule goes.
However, take a look at the instructions of the agency that doesn’t want you to use private delivery services. Some agencies like the IRS, USCIS, and the Department of Education that lots of paperwork come through have separate mailing addresses for private delivery services. As they are almost always faster than the USPS and have express shipping options, you can utilize them if late for paperwork.
The bottom line is that any delivery service that’s not the USPS is private, and you should only ship certain things through the private delivery services where you’re allowed. Otherwise, your package or envelope will be returned to you.