Interestingly, I just discovered that to open the box, you never even have to go in to the same one you desire the box at. You just have to provide the proof at a post office. (That would have been convenient for me to know!) Bolding (and underlining) by me.
USPS.com FAQs
How do I apply for a PO Box in person?
To apply at a Post Office, you must complete all required information on PS Form 1093, Application for Post Office Box Service (to view and print the form,
click here). If you apply online, this form is generated during the application process. Bring a copy of the completed form along with two forms of acceptable U.S. identification (
one must be an acceptable primary of identification and the other an acceptable secondary form of identification) for verification to a Post Office that provides window service. See
Acceptable Forms of Identification.
The Post Office need not be the one where box service is desired. Please note that furnishing false information on the application or refusing to furnish required information is sufficient reason for denial of the application or discontinuance of service.