Married here 21 years ago, I provided the "Affirmation" which was translated & then certified by MFA which was kept by the Amphure, and subsequently "mislaid" by them, Head of Amphure put that in writing to Immigration but to no avail.
I had the money in the bank already, so the IO halved the pile of submitted papers, gave the rest back, approved Retirement and stamped passport, did not need to do wife & witness interview's, etc., nor submit to higher authority. No agents in locality, and not sure I would trust doing by mail.
Separate issue but mail posted in Thailand can take 6/8 weeks to be delivered if it arrives at all, about the same as post from UK.