It’s often the little things that make a difference. When I checked in online for my flight from San Francisco to Boston I discovered that the one bag I planned to check would cost me $15. I haven’t been living in a cave so I knew about the plans various airlines announced last year about charging for checked bags, but this was the first time the fee had been applied to me. “Nonrefundable,” the note on the computer screen said. “Forget it,” I muttered, figuring I’d wait until I got to the airport to decide whether to check or carry on.

I happen to detest carrying luggage onto airplanes. In my darkest moments I could probably confess to detesting those who do carry their bags onto airplanes, but it’s only those who bring along their spare sofas disguised as wheeled luggage who truly bother me.

But I also am no fan of standing around a luggage carousel waiting for a bag to magically appear, so I do wheel on my rollaboard from time to time. These days, though, with the regrettable proscriptions on liquids in containers larger than three ounces, I usually just check my bag to avoid the hassle of trying to smuggle a four-ounce bottle of contact lens solution on board.

But $15 is $15, right? Well, the friendly skycap at SFO seemed chagrined that he had to charge me, and he couldn’t take cash, only credit cards (what, United doesn’t trust their hard-working, always-smiling, ever-efficient skycaps to turn in the cash?), not to mention that his tips probably go straight out the window when passengers have to pony up. But I decided in the end I’d rather not hassle with the bag through security, and I gave him a cash tip for his trouble.

Then I looked at my receipt and there it was, plain as day, United’s vaunted customer service culture staring me in the face: “Excess Baggage Ticket Customer Receipt” it read.

I breathed an expletive. Excess baggage indeed.

At the gate I saw the usual gathering of sofa-wheeling passengers, some with toasters, one actually trying to stuff a four-burner stovetop range into a black canvas bag designed to fit in the overhead bin.

My boarding pass had a big “Seating 4” boldly printed on it, meaning that I’d be in the last group to board because I had an aisle seat. I didn’t mind, all I had was a small computer bag with a couple of books. But then the gate agent announced that the plane was filling up, and all passengers in number four seating group with rollaboard luggage should come to the counter to check in their bags because there wouldn’t be room on board for them. “Do it now,” she said, “so you don’t have to come all the way back from the jetway when you’re told there’s no room.”

Amusing, I thought. I’m sure United’s bean-counters didn’t think about this when they imposed their $15 checked-luggage fee. Not only are they forcing people to carry on every last piece of luggage to avoid the surcharge, now they’re going to have to do battle with livid passengers being told they can’t carry on their bags, they need to check them here at the gate, and each one will be $15, please.

Imagine the uproar. We’ll never get to Boston, I figured.

Except there was no charge.

After observing a half dozen friendly — and FREE — transactions, I approached the agent and said, “If I had brought my bag here to the gate instead of checking it in at the curb I would have saved $15?”

She smiled. “You sure would have!”

This is why I love United Airlines.