Is using something like the Square Reader allowed for PTAs?

QUESTION: Our PTA is discussing the possibility of using a Square Reader for credit card purchases at our big events.  We currently use another provider that charges us monthly whether we use the machine or not.  Square Reader has an option to charge just when you use the reader, by transaction, which would save us money.   Is using something like the Square Reader allowed? ANSWER:  We love using the Square for our events. The Square has upfront pricing, uniform pricing and no equipment rental! For more information, click here.