Please answer the folloiwng questions
Any other formal words for "then"?

If you agree or disagree then let me know.
In above sentence does word then is formal or any other words.

If you agree or disagree "so" let me know.
What do you say about word "so"?

Question

Please answer the folloiwng questions
Any other formal words for "then"?

If you agree or disagree then let me know.
In above sentence does word then is formal or any other words.

If you agree or disagree "so" let me know.
What do you say about word "so"?

Answer

You could omit the word "then" without losingthe meaning but adding "so" doesn't really make sense.

Answer

In the context of that question you could also replace then with please. So it would be:
If you agree or disagree please let me know