TLDR : Answer Summary
Yes, the 60-day COVID extension is available for US passport holders on a SETV before January 29th. To apply for the extension in Samui, you will need several documents: a copy of your passport data page, a passport photo, proof of filing a TM30, a copy of your departure card (TM6), a copy of your latest entry stamp, a filled-out TM7 application form, a filled-out STM2 form confirming your understanding of overstaying rules, a filled-out form indicating your wish to stay due to COVID (exact name unknown), and a fee of 1,900 baht. Note that requirements may slightly vary by office.