And often the credit card companies try to enforce this by writing wording into their contracts that try to stop merchants offering different prices for different payment methods.

This is true. In fact, if you come across a merchant that accepts credit with a minimum purchase amount or tacks an extra fee, you can take the receipt and call the terminal owner (visa/MasterCard) and report it. The receipt has a terminal ID, and it turns out the likes of Visa get really pissed if merchants do that.

I know this because I used to work on a US military base. There was a sole merchant on a particular installation that was doing this, but it was extreme. They'd force you to buy over 10 USD if you wanted to use a card, and it was the only place to grab a snack. One day there terminal was shut down, and for a period they only went to cash payments. Once they took credit again they removed the minimum purchase. Turns out somebody got pissed, and reported them.

Regarding the article, the ~390 USR sounds about right to me. I only use a credit card, pay it completely off at the end of the month. I've never once paid interest since in the 8 years I have used this card. Every few years I buy a plan ticket with the rewards.