An Arlington woman writes: "Last time I wrote to you, it was a story about nice people. This one isn't.

"On Sunday, I locked myself out of my car. I called a mobile lock service for help. Their man was in Rosslyn at the time, and I asked for him to come to Pershing Drive and Glebe Road, about 10 minutes driving time.

"It took him 5 minutes to open my car, and he charged me $35. I'm a widow and I was shocked. When I told the man I thought his charge was outrageous he said it was Sunday and I ought to consider myself fortunate to get a locksmith to help me. Don't you think his charge was highway robbery?"

No, I don't. I think it was high, even for a Sunday, but not really "highway robbery." Unfortunately, fees for most services are not set according to the customer's ability to pay, so the impact on a person of limited income is sometimes quite severe.

To protect yourself against the cost and inconvenience of being locked out, you ought to make it a practice to carry a duplicate set of keys at all times. I would no more think of setting out in my automobile with only one set of keys than I would with one shoe.

Scattered through various right-hand trouser, jacket and overcoat pockets, I carry keys to my house, office, desk, file cabinet, car ignition and car door. Also ignition and trunk keys to my wife's car, just in case there's sudden need to switch. In my left-hand trouser pocket, I carry one key ring on which there is a duplicate of every key in the right-hand pockets. I used the right-hand keys every day. I use the "reserve" key ring about twice a year when I mislay the keys I use every day.

This simly precaution has not only saved me a lot of money but the embarrasment of finding myself absent-mindedly locked out. I strongly recommend the idea, even to those who can afford to pay $35 to get their car doors opened.