A basic rule of economics is that the price depends on how willing consumers are to buy something else.

The less consumers pay attention when they buy — and the more they just follow a set shopping pattern — the greater the market power possessed by the seller, and the more that seller can charge.

Modern companies know this very well, and they do what they can to improve their advantages over unwary consumers.

I’ve unwittingly been caught in consumer traps myself.

About 15 years ago, for example, my wife and I decided to send out holiday cards using an online photo service rather than writing them out and sending them by hand. We picked Shutterfly, which did a good job. And we’re still using it, largely because after we had typed 100 addresses into the Shutterfly site, it would have taken a heck of a deal to persuaded us to type them in again somewhere else.