Apologies for perhaps making a correction, the Post Office do not deliver mail, Royal Mail and ParcelForce do, and even those are two different companies.
Regarding leaving mail at someone else's address, Royal Mail (so they tell me) have very strict rules about this. Their delivery people are not allowed to do this, including following instructions posted on doors and in windows etc. I have a written arrangement regarding leaving mail and parcels elsewhere, the local Royal Mail new management stated that I hadn't and couldn't possibly have this agreement, after one replacement driver refused to leave an urgent parcel via instructions. Funny really, when I produced a signed agreement from an higher level management structure, open mouths all round!!.
The local management then apologised, stating that due to a refurbishment and restructuring exercise, my agreement posted on their sorting frames had been lost.