The indefinite article a refers to “mythical description” in your example sentence.
“This is the result of a—in my honest and sincere opinion—mythical description of human nature.”

You could also rewrite as follows:
“This is the result of a mythical description of human nature (in my honest and sincere opinion).”
“In my honest and sincere opinion, this is the result of a mythical description of human nature.”
“This is, in my honest and sincere opinion, the result of a mythical description of human nature.”

“This is the result of a/an — in my honest and sincere opinion — mythical description of human nature.”

Your sentence is correct as written.

As the post states, “In knowing when to use a or an, we should also consider how the following word sounds.” In other words, the sound supersedes the letter in using a or an. We would use a before words beginning with a consonant as well as y and w sounds. The word “use” starts with a y sound, so it would be preceded by a.

As the post indicates, use an before a word that begins with a vowel sound. Therefore, write “…an Agreement…”

Provided that a Agreement for Payment is received….
or
Provided that an Agreement for Payment is received…

Initialisms are pronounced with a starting vowel sound (“em”-bee-ay) or starting consonant sound (“eff”-bee-eye). This will influence whether to precede the initialism with “a” when the first following sound is a consonant or “an” when the following sound is a vowel.

Kristen got an MBA degree.