Delivering parcel to a P.O. Box address in UK

Having had received an order by an internet bookstore near Liverpool, I sent by An Post a parcel of a few books to a P.O. Box address displayed on the bookstore's website. However, upon enquiring if the parcel had arrived, I was told that it didn't, because it's a P.O. Box address. The person who ordered (and neglected to give me an alternative address for the parcel) said that parcels should never be sent to addresses depicting "P.O. Box xx", which is for mail only (I suppose he meant letters only). That's news to me.
Did any of you ever send parcels to P.O. Box addresses (of which I had no knowledge of).
Allendog

Some companies don't/won't deliver to PO box addresses AFAIK

Normally when a Company uses a P.O. box number they pay a fee which covers delivery of letters only. They are given the option of paying extra to facilitate the delivery of parcels but that option is never taken. Not sure what the regulations are in Britain re returning parcels but it should come back to you if undeliverable. You should contact An Post, fill out an enquiry form and keep following it up. You should have got a reference number when posting - this is important for tracing.