I'm not sure if you need an address to sign up for a private mailbox at places like UPS Stores.

But a lot of people might receive mail at a friends' address with permission. But, you still need to have a friend or family with a stable address who is willing to help.

In the past this was pretty lax (I've had a long-term box at a Mail Boxes Etc. that then became a private mail boxes place that then became a UPS Store) and they didn't really care when I first opened it. Now there's a push for KYC also; we got a sheet the other day asking to verify our physical street address, something I never personally got in the years I've been there. Apparently new regulations or something, they said.