There is not much to shop for in San Pedro beyond wool and stone, and that is fine, because the wool is genuinely useful. Alpaca and llama knitwear from the craft market keeps you alive on a pre-dawn minibus in a way that whatever you packed probably will not. Lapis lazuli, mined in Chile and almost nowhere else, is the other thing worth buying.
Bring cash. The two ATMs in town run out with some regularity, especially at weekends and month-end, and a surprising number of places still do not take cards.