What is the process of applying for a 30-day extension at MTT using a US Embassy letter?

Wanted to share my experience today at MTT as I used a US Embassy letter to apply for the 30 day extension. Arrived a bit late for my 1pm appointment due to taxi driver not being able to find the facility (luckily I was able to use the pin from an earlier post to navigate, otherwise I might still be driving around). Personnel at the front check in were very helpful and directed me upstairs to the parking deck waiting area. In contrast the attendant there was flustered and would only direct me to take a seat and wait but was unable to tell me if I could check in anywhere for my online appointment.

Waited about 45 minutes until my turn came and presented;

-passport

-photo

-copy of passport info page

-copy of arrival card

-copy of arrival stamp page

-two filled out forms that I received there (gave my particulars, acknowledged the penalties for overstay, and a brief reason for my extension request)

All papers were checked, stapled together, returned to me and I was given a number to take to the office downstairs.

Waited downstairs for about 20 minutes then my papers were checked again and I paid 1900 baht. Directed to a third window where after another 10-15 minutes I received my passport back and saw the below stamp telling me I was under consideration for the extension and that I should return on the 16th of October. The last attendant was very helpful but did not speak English, though I managed to gather that I should make an online appointment and return any day around the 16th but no later than the 18th. Sounded strange but I left telling him I would be back on the 16th as stamped.

Thanks to everyone in this group, all the information here made a confusing situation easy to navigate.