Overdraft charges are among the first financial penalties a young account holder meets. Their structure, rather than their size, is what produces the damage.
The charge attaches to the transaction, not the amount
Institutions generally assess a flat fee each time a transaction is paid against insufficient funds. The fee is the same whether the shortfall was a few dollars or a hundred.
That structure means several small purchases made after a balance runs out can each generate a separate charge, and the total can exceed the amount actually overdrawn by a wide margin.
The purchases themselves are usually ordinary and small, which is why the outcome feels disproportionate to the behavior that produced it.
Posting order determines how many fees occur
Transactions do not settle instantly. They are processed in batches, and the order in which an institution applies them within a batch changes how many are paid against a shortfall.
An account holder watching a balance in an app is seeing an approximation, since pending items, holds and deposits that have not cleared are treated differently by the display and by the ledger.
The gap between the displayed balance and the available balance is where most unintentional overdrafts originate, and it is not visible to someone who has never been told it exists.
Opting in is a separate decision
For certain transaction types, an institution generally needs the customer's affirmative agreement before covering a shortfall and charging for it, and declining means the transaction is simply refused.
Being declined at a register is unpleasant and free. Being covered is convenient and priced, and many people agree to the second without registering that they chose it.
Alternatives are priced differently
Linking a savings account or a small line of credit as backup usually costs less per event than an overdraft fee, though terms differ substantially between institutions.
Some accounts, including many at credit unions and some designed for students, carry no overdraft program at all, which removes the mechanism rather than managing it.
Why the burden is uneven
Fees fall on accounts that run close to zero, which means the cost of this product is paid almost entirely by people with the least room for error.
Rules governing disclosure and consent are set federally and by state, and they have been revised repeatedly, so an account's current terms document is the authoritative source rather than general description.
Anyone facing repeated charges can ask the institution directly about waiving them and about switching to an account without the feature, both of which are ordinary requests.