A P.O. Box is not an acceptable proof of residency. Documents mailed "in care of" are not valid for proof of residency.You could write a letter or a declaration under penalty of perjury that attests to the person's residency. I wanted to know what documents would serve as proof of residency - Electricity Bill? Download a proof of residency letter template for family members, notarized or non-notarized, tailored for legal use. A proof of residency letter is an acknowledgment or sworn statement stating that an individual resides at a specific address. The RMV itself does not require any proof when registering the vehicle.