You've gotten good advice from others but note that Peru has a highly dollarized economy in the sense that you can pay for almost anything, anywhere, in dollars at a fair exchange rate currently around US$1.00 = S/.3.00. Also there are lots of moneychangers hanging around the banks, and they charge no commissions and very fair exchange rates. Only day that may be a problem is Sunday when there are fewers moneychangers around and they may change the exchange rate.