withdraw! A nice USCIS officier shouldn't be so cruel to reject your petition because of the overpay. But now with so many I485's before Oct. 1, anything could still happen. So you will have to decide whether to make another I485 submission or not. If I were you, I would just wait.
One way to check if USCIS has accepted your package is to check your bank to see if the check has been cashed. Since you submitted on last Thursday, your check is likely to have been cashed if your bank is a bigger one.