No, it is not mail fraud. Call the attorney and ask for an explanation. You do not know why the hourly rate was changed. It could be a computer problem or perhaps you were billed at the wrong rate the first time the bill went out. We cannot read the attorney's mind. If you have an engagement agreement with the attorney, review it and be prepared to discuss it.
It's not mail fraud. Call and ask about the billing mistake. Or pay the lower bill if you think it's not a mistake. Sloppy billing is rampant and seldom sinister or intentional. If you are certain this is deliberate over-billing, pay what you think you owe and call the ARDC.
So far, this is free to you. Until you pay a fee, I am not your lawyer and you are not my client, so you take any free advice at your sole risk. I am licensed in IL, MO, TX and am a Reg. Pat. Atty. so advice in any other jurisdiction is general advice and should be confirmed with an attorney licensed in that jurisdiction.
I agree with my colleagues that the easiest and most effective way to deal with this is to simply ask your attorney. Also, your retainer agreement should spell out the hourly rate(s) and manner of billing as well as how to contest any billing errors. If you do not have a written agreement, you need to have one ASAP, for your sake as well as the lawyer's. Good luck.
I have been licensed to practice in the State of Oregon since 1990. I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.