I was recently told by the a M and T vice president that "the system" determines which checks/web payments/debits are allowed to process . The amount in your bank account is not a factor in the decision as you the account holder are ultimately responsible. The bank then processes the check/debit/web payment and charges insufficient fees as a courtesy to you. The amount of the insufficient fees varies depending on the branch where you opened your account. From what I understood, each branch has flexibility in whether to refund fees and the amounts of fees charged.Every morning the "system" reviews accounts with negative balances and decides whether to process the transactions that generated the negative balance. The largest transaction, which results in the negative balance, is processed first and will not be denied.
It is important therefore for every one who is not able to understand this "system" to call/visit your branch manager to discuss any issues directly with M and T.
