In reply to Red Rover:
Post Offices in the Highlands may have limited opening hours. If you're passing when they're not open it's a bit of a pain having to hang around until they do. All sorts of things can throw you off your itinerary.
Its' not for me to say which people near your route would accept a parcel for you, but a courteous 'phone call or email to somewhere you might stay or buy something from is the way I'd play it.
I've never had the request refused, and have met some wonderful people this way.