I know it is user error, but it sure would be nice to have something to prevent the user who is not paying attention from selecting UPS or FedEx to a PO Box.
Even with shipping rules to set it to a default of USPS, our shipping people look for the lowest cost and select FedEx or UPS to a PO Box. It happens a couple times a month (we ship several thousand packages every month).
We end up paying the outbound shipping, then paying for the return shipping + a second shipping charge to send it the right way or a $15 address correction. Seems like it should be rather easy to flag these and pop up a warning or just simply not allow it....