Non-USPS mail services use physical addresses. The post office delivers all the mail there, and it's the responsibility of the private party to sort.

Therefore you can list the "box" as a "unit" number, or "ste" (suite) or "apt" (apartment). Ask the 3rd party service prior to contracting.

I had such a service in NYC (they're common, lotsa businesses love having a Suite address on 5th Ave, or whatever).

I'd test it before committing to a long term contract though.