Well, no replies to my question! We did use the service however and here's how we found it...
3 post offices used on our May 3 week trek from Torridon to Cape Wrath taking in munros en route. Post office service was excellent with the only hiccup being at Kylesku where we hadn't realised the bank holiday would be adhered to!
Phone up to confirm arrangements - we took the website published opening times for Ullapool P.O. as gospel only to find at the last minute that it closed at 1pm on Saturday! A mad rush across Loch Broom with 5 minutes to spare!
Also, a thank you to Lochcarron Garage who, at short notice, started running a replacement for the Torridon Post Bus service after the service was cancelled.
Now to start planning the next trip...