All the private courier companies (DHL, Chronopost, FedEx, UPS, Yodel, etc) cannot deliver to PO Boxes because they offer a "signed for" service.

If you only have a PO box, you must enter a valid phone number. Please also note if this is the case, then your goods will not be delivered within 24h, as the courier will have to give you a call to arrange for you to pick up the parcel, or deliver to an alternative address.

All the other services we currently offer (Royal Mail First Class, Colissimo, etc) go through the "normal" post offices - the national postal services: Royal Mail for UK, La Poste for France, AnPost for Ireland... - therefore can theoretically be delivered to PO Boxes as some items might be too large to be left in the box.