PHILADELPHIA - If you haven't done it, you're among the purest of the pure: someone who's never bounced a check. Most of us who just muddle along have had these embarrassing experiences, not because we're cheats but because, well, we screw up.

You blew the math in your checkbook, a deposit took too long to clear, you and your spouse didn't talk, or perhaps someone bounced a check "to you."

No matter how innocent the mistake, most people get a hollow, guilty feeling and want to slink into a hole. This summer I wrote a bad check to a kid who'd done chores around the house. In a rush, I'd grabbed a fresh book of checks - one from a defunct account. What a sinking feeling when my employee's mother called. (He was at Scout camp.) Explaining just made me feel sleazier.

Usually a bounced check costs you a fee of $30 or so. The bank's got to cover its costs for fixing things, right?

Not exactly. The Consumer Federation of America, using data from the American Bankers Association, has found that banks make a bundle on returned checks.

The average bank charge for a returned check is 84.4 percent more than the cost of handling the situation, according to The Green Sheet, a financial-services newsletter. Markups are bigger at bigger banks, with banks of $1 billion or more in assets charging nearly 10 times as much in returned-check fees as the processing costs.

Obviously, if you don't bounce checks you don't have to worry about the fees. Keep your checkbook balanced. And don't forget those insidious withdrawals. Some people keep large balances in their checking accounts to cover mistakes. But checking accounts pay terribly low interest rates, or none at all. Same problem with overdraft privileges that draw from savings accounts.

My approach is to keep reserve cash in a money market account with my discount broker. It pays a better interest rate, 4.5 percent, and it comes with a checkbook.

For regular expenses such as the mortgage and electric bill, I still use a bank checking account, since it's more convenient and returns the canceled checks. I keep just enough in it to cover predictable expenses, using a plan that gives me free checks with a low, $600 minimum balance requirement. I have no savings account.

Because there's little margin for error, I occasionally double-check my balance by phoning the bank, but I don't take what they say as gospel, either.

On the same occasion I bounced the check to the Boy Scout, I'd written a couple of checks to myself to shift some money. The bank assured me they'd cleared, but they hadn't. Fortunately, in this case I kept a margin to cover the questionable checks, having learned from an experience a few years ago when I rented out my house while I was working out of town. On that occasion, when the tenant's rent check bounced, it took weeks for the notice to reach me. By then I'd written my mortgage check against the tenant's "deposit."

Which brings me to the final point: Trust, but verify. When my mortgage check bounced, the lender sent a snotty letter imposing a $160 penalty. I called to say the mortgage specified only $40. "Oh," said the rep. "You have a good history, so we'll waive the penalty."

Great, but what if I hadn't called? A paranoid might think the lender overcharged on purpose, knowing most people feel so guilty they'll pay without question.

Or maybe they'd just made an honest mistake. But it goes to show that if something goes wrong, you shouldn't just assume that "you" are the bad guy. Feel guilty - but don't pay the bums more than they're due.