Should I list a letter in my cv? If someone writes a letter to the editor and I write a response, how should I list the response this in my CV? 

It is peer reviewed but is only a 300 words, so it doesn't seem right to put this under 'journal articles' (though the title starts with "Reply To: ... ", which makes it clear).
I put these on my CV as peer-reviewed publications and label them as 'commentary'. I also label other works 'review', 'empirical', etc. as appropriate. The title can also be used to signal the type of work, like "[Title]: Commentary on Zeller et al., 2018".