How did you keep from shouting "Are we really arguing about 90 cents?!"
Back when I was still working loans/lines I had a banker call in for a customer. The banker knew what was up, but the customer was being belligerent, so she was just calling to appease him. Anyway, the issue was that there was an interest charge on a line of credit. The customer's overdraft protection had kicked in and he had paid it back the next day. Now, most lines of credit, unlike credit cards, have no interest free grace period. Interest begins accruing the day the customer takes out money against the line. The total amount of interest this customer had to pay was five cents. This guy spent more money in gas driving to the bank to complain about it. He got nasty about it, yelling at people to the point where I could hear him clearly even though by that point the manager had taken him to a closed office somewhere else in the branch (the banker had kept me on the phone to ask about a real issue with another customer's account). Unfortunately, the phone call ended when my banker told me he was going to have to hit the panic button because of the original customer's behavior. Over five cents in legitimate interest charges. The funny thing is if he had been nice about it, the banker would have probably waived it.
Oh I could totally see my friend's DH arguing about this! He is cheaper than cheap, have never ever carried a balance on a credit card (where you'd have to pay interest), and paid cash for the ONE car he's ever owned.
Before they were married but living together, they got a joint cc, with a period of no interest. Which ended after so many months. Well, he didn't pay attention to when it ended, they had a balance, and got charged interest. He had a coniption! screamed and yelled, and carried on, and was told, hey, you agreed to the terms, so its your responsibility to make sure you pay it off, if you don't want to pay any interest. He was not happy, but paid it. And I think it was less than $10.
For some people, it is the principle, but they also obsess over stuff. Esp stuff they have no control over, such as gas prices, and so on. Her DH is one of those.