In the United States:
Yes, we ship to PO Boxes via USPS Priority by default to ensure successful delivery. This is because FedEx does not reliably deliver to PO Boxes within USPS Post Offices, and USPS Post Offices won’t reliably accept deliveries from FedEx (due to space constraints, etc).
If your mailbox is in a private mail business (as opposed to being within an official United States Post Office), FedEx is more likely to successfully deliver your package, in those cases we'll use FedEx as the default carrier.