question about financial fraud?

about a month ago i went to my foster parents house. I saw a letter on the counter and it was a check. I used the routing and account number off one of here clients check(254.45) to pay a cell phone bill. About a week later i called and apologized and reversed the payment back to them from verizon and all the money was paid back. then another week later i get a call from a detective and i have and arrest warrent out for financial idenity fraud. This is my first offense for me not even a ticket. How much trouble can i get in for this?
i live in south carolina

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