You may also be interested in these guides: Does USPS Require A Signature For Delivery? and How to Use USPS Signature Confirmation Effectively with eBay and Paypal
Yes, if you ship an item with Signature Confirmation to a Post Office Box, the recipient will receive a notice that there is a package that requires a signature. The recipient will then take the notice to the Post Office window and sign for the package. This is exactly the same process that would happen if you shipped Signature Confirmation to an address and there was no one available to sign for it. For my guide on when Signature Confirmation is required on eBay and Paypal see, How to Use USPS Signature Confirmation Effectively with eBay and Paypal. Remember, only USPS can deliver to Post Office boxes so if you’re shipping with UPS, Fedex, or anyone else it will not be possible to ship to P.O. boxes.