The only carrier that will deliver to a PO Box is USPS. The other major carriers, like UPS, FedEx, and DHL, do not deliver to PO Boxes. These carriers do not have access to your PO Box; therefore, they cannot leave mail or packages in your box. If you have a package delivered by USPS that is too large to fit into the box, they will typically leave a notification card in the box stating that you have a package held for pickup.