A valid Proof of Address must state a residential address, not a P.O. Box. However, there are still ways to get approved if you only have a P.O. Box document:
Submit another government-issued ID with your residential address, or
Obtain an official letter from any of the following:
Bank manager
Magistrate of a District Court
Landlord of rented premises where you live
Customs or Immigration Officer
Notary Public
Police Officer or Commander
Accountant who is a member of an association of accountants
The letter must:
State that your country/locale does not have a residential address system, and/or
Confirm your actual residential address.