This doesn't sound like the end of the world. Hire a lawyer and see if the DA will dismiss the charges since you've paid the money back, or perhaps agree to dismiss the case after a short probation period with the promise to dismiss the charges if you stay out of trouble.
You need to hire a lawyer. While the amount of money is not much, it is two offenses (each card) and could be charged as a federal crime because it involves mail. Do not talk further with police until you have a lawyer to advise you.
Get an attorney! In Philadelphia this is an ARD case at best. I've seen folks not show up at preliminary hearings for Rape and other assault cases. I can't picture anyone showing up for $75 worth of gift cards.
Don't risk the Preliminary Hearing without an attorney there are a lot of ways to make this go well but just as many to make it go badly for you.