One of the best things to buy on South Uist is a package of Salar Salmon. It's available at the factory shop in Lochcarnan as well as a couple of shops throughout South Uist. The company also runs a mail order service. Makes for a great lunch sandwich!
There's a fairly new Co-op Supermarket in Daliburgh said to be 50% larger than the previous one it replaces. Apart from that one, there's a small shop in Howmore and another larger Co-op in Creagorry which is already on Benbecula.