Sai - a good question. They can mean the same thing. However, for me, a commentary relates to feedback related to a specific article. A letter to the editor can be the same thing - but might be related to a broader issue related to the journal overall.
Firstly, thanks for your response with regards to my question. I completely agree with your thoughts on commentary and letter to the editor articles. However, from my understanding, in addition to Dr. Dean's answer, 'commentary' articles could also be a short/mini reviews with lack of evidence which needs a feedback/comments from the readers. (innovative ideas which adds to the current body of literature)
Whilst, 'letter to the editor' could also be a kind of corrections/appraisals of the articles that were published in the current/past issues of that journal. (updates)
Correct me, if I am wrong or appreciate, if I am right.