Is it rational for someone to choose to NOT buy another mattress, not because of the sunk cost, but in order to "punish" oneself (stick and carrot style) to change their behavior and not buy non-returnable, expensive things, ever again?

The key question is: Why do you believe that this attempt at punishing will have this effect?

Is that a theory that you came up with yourself as a person who isn't an expert in psychology and who hasn't read any of the relevant research or alternatively has done enough self experiments to have empiric data about how this effect will work on them?