the same when using a CC to make a purchase in €...sometimes the POS CC reader will ask if you want the charge to go through in € or USD, and if you choose USD the exchange rate will be terrible...much better to have the transaction processed in €, and let your bank do the currency exchange when it hits your account

while there is no reason to "purchase" euros in the US before traveling to SXM, it doesn't hurt to get some from a french side ATM after arrival, assuming you gave a debit card that doesn't charge ATM fees and reimburses those charged by the ATM (ie. Fidelity, Schwab)...especially if you will be visiting SBH, where USD is not widely accepted