St John's is beautiful but as far as shopping goes you are slightly limited:
-Water Street has cute boutiques and there are 2 malls.
-Sears sells Clinique, estee lauder elizabeth arden and clarins
-There is a shopper drug mart with a beauty boutique that sells benefit and guerlain and other brands
-I dont know of anywhere on the island where you can buy MAC or Chanel makeup

but you can always order online.
Any other questions let me know, I dont live there but I go for a month every summer to visit family.