Correctiong the service tax charged

Since you have charged amount which was not required to be charged, You should return the service tax component to your clients otherwise It will be credit in government Account. I suppose that you must have deposited service so collected to the government. If it is so, then either you can make refund application only if you return the service tax component to clients

Further for charging lesser rate, since the amount is not very substantial then you can take the amount received from client inclusvie of service tax and make calculation accordingly..........