I'm on study leave and have heard the postman knocking on their door from time to time. Should I mention the above situation to him and offer to take in their post? The parcels are usually delivered by an outside courier but the postman delivers their usual post i.e. bills etc. It would save them a trip to the sorting office to pick up their post as well.
EDIT: Would like to add I live in a block of flats and their flat is across the hall from me. Also they are not very well sound proofed so I am not stalking the postman.