We always load our packs with several kg of chocolate (mainly to be used for making hot chocolate) at a small shop in SdC before returning home to Barcelona. I can't tell from the photos if it is the same shop shown in the images provided by Gerard Murdoch or if it in another shop a couple of doors away. Unfortunately, you are unlikely to find any chocolate of interest in the supermarkets as they tend to stock only mass produced (i.e., interior) brands. You will, no doubt, find interesting, artisanal products in the small food shops in SdC, perhaps even some of the famed Astorga chocolate you mention. If you have room in your carry-on luggage, you might also consider buying one or two Santiago tartas to take back home. Furthermore, you might think about purchasing some of the lovely Galician liqueurs which are far better than anything you will find for sale at the Duty Free shop. A couple of favourites for me are the chocolate and cherry liqueur and the crema de orujo. Local tinned fish (e.g., octopus, sardines, muscles, tuna, etc) also makes a great souvenir and is easy to transport.
