Your Customer would need to submit a proof of residence with their PO Box on it. If the Customer does not have a proof of residence (POR) for any reason.
This is a great Tip: You can have him send a self-addressed stamped envelopes to himself. Once the Self addresses stamped envelope arrives he should scan in or photograph the envelope that the consumer receives with the US Postal Service Mark on it. This can serve as a proof of residence, it demonstrates that if a letter is sent to himself the USPO delivers it accordingly.