The Eastern Caribbean Dollar is the legal tender in St. Lucia. The exchange rate is 1USD to 2.70XCD. Banks are usually open from 8am to 3pm Mon to Fri and closed on Saturdays. All major credit cards are accepted at most places so travellers shouldn't have any problems whatsoever.  Travelers checks are a bit more difficult as many businesses do not accept them.  You will need to either exchange them at a Bank or, possibly, at the resort you are staying.