IBAT just published an answer to this question.
"I'm concerned that some Texas banks might think that the Fed is approving a daily fee. Alternatively, some banks may adopt the Fed's example without noticing that the fee is in the example or without knowing that charging this fee may be usurious.
The Texas case that we call Tony's Tortilla Factory (because that's the name of one of the parties) holds that a one-time charge on an overdraft is not interest because the formula for interest is:
PRINCIPAL X RATE X TIME = INTEREST.
With a one-time charge, there isn't principal, rate, or time; therefore, there is no interest.
If instead you charge $5 per day after the account is overdrawn for 5 business days, you do have principal (the amount of the overdraft), time (one day or more), and rate ($5 per day). If $5 per day is more than 18% per year, then it is usurious. On a $50 overdraft, you could charge $0.02 per day.
By my calculation, in Texas, you may charge $5 per day for overdrafts, but only if the overdraft is $20,277.76 or more."