PO Boxes are owned by Canada Post, and only Canada Post has access to be able to deliver to them/has the keys for them. For this reason companies other than Canada Post must forward your parcel to Canada Post at the final destination.
For example if Purolator is selected and the final destination is a PO BOX. Purolator will take the package as far as possible and forward to Canada Post for final delviery.
Due to the forwarding costs associated with this - your shipping service is subject to change unless the additional fees are covered by the customer.