You insert commas before and parenthetical expressions, which are words inserted into a sentence giving information that isn't essential. The commas are used correctly in the examples you provided, but there are other ways to write it. For example, if this a letter or email then it would be redundant to write the recipient's name within the body of the letter, so you can safely drop it from the sentence. Another option would be to break them into two smaller sentences: "Thank you for writing me back, Michelle. All the best to you in 2019!"
When to use mid-sentence commas when adding a formal name?
3 Answers By Expert Tutors

Mary M. answered 03/22/19
Lifetime Experience Helping All Ages with Grammar Needs
In general, grammatically, if one can take the name within commas out of the sentence, then adding the commas is acceptable when addressing that name directly. The second sentence might be construed by Michelle as judgmental, so I would omit that sentence after the Michelle commas, adding another qualification. Our discussing your individual expectations should help you locate a tutor who is 'in-tune" with your specific needs and wants. Sincerely,..I wouldn't repeat the two sentences you noted together, since they are slightly redundant (my opinion).

Hien B. answered 03/22/19
Oxford Grad | Experienced Writer & Editor
The use of commas in each of those examples is indeed correct!
As a rule-of-thumb, it may be helpful to say the sentence out loud do to see if it makes sense with the commas or not. I also think it'd be helpful to just think about each comma individually instead of starting by asking if there are too many commas. If it makes sense to use the comma before, then use the comma before. If it makes sense to use the comma after, then use the comma after as well. Each use of the comma is justified on it's own, and so it follows that the use of both commas is justified and that this isn't a case of too many commas. There can only be too many commas insofar as the use of an individual comma is inappropriate.
Hope that helps!
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