It depends who the carrier is. If its small and postal carriers are delivering then yes. If its a larger item and a carrier like ups, FedEx or a trucking company then you can't deliver to a p.o. Box. Most companies will say on website when you're filling out address if it can deliver to a p.o. Box.
You cannot send a package to a P.O box from a website. The P.O box is only meant for the physical packages and you have to have it stamped for it to be sent. The website is only used to send email (electronic mail).