There was a recent thread somewhere that suggested that the use of $ for pesos was related to the one-to-one pegging of the peso to the US dollar during the 90s. There is a peso symbol, but everyone has been using the $ for so long, that is was is used.
Always verify. Our shipping company tried to charge dollars, even on repeated questioning, until a accountant somewhere slipped up and wrote on a receipt "... y ZZZ centavos". On further questioning, we got the "Oh! I didn't know that! So sorry." Fortunately, we hadn't paid anything yet. (Lift Van International for anyone interested...)