SoFi doesn’t charge any. If you write a check with insufficient funds or try to withdraw an amount that exceeds your balance, the transaction will not go through and SoFi will not charge a fee.
While SoFi Money prevents you from overdrawing in most cases, there may still be times when you could have a negative balance. This could happen if a transaction is approved for one amount, but the actual charge is more than the amount of money in your account. For example, when you use your SoFi Money Debit Card card for a meal and then add a tip. In this situation, the full payment may go through and result in a negative balance, but even then, SoFi will not charge you an overdraft fee.