First, did you get the list of required documents from your Amphur?
Some like Mae Rim don’t require it.
Second, do the requirements specifically ask for your passport to be authenticated by the Thai embassy/consulate?
Either the embassy in Bangkok or the Consulate in Chiang Mai. It’s a 3 minute walk in $50 USD service.
The real hassle is getting the certified copy translated and then authenticated by the Thai Ministry of Foreign Affairs.
Pro Tip: make sure you are happy with the translation of your name into Thai because it’s very difficult to correct the translation later after it’s on 50 other documents and ID’s.
You should probably hire a competent, reputable agent for your first attempt.
Copy or photograph all the forms so you can renew it yourself next time.
It should cost no more than ฿15,000 THB all in, including a multiple re-entry permit and all other fees. it will be initially valid for 90 days. Then, you renew it annually.