Thank you, tjintx, Bill97, and connda, for your input!
I received my passport today, 1.5 weeks after it was mailed by the U.S. Department of State.
The prepaid THP envelope I purchased included a tracking number, which would have been easily trackable if Thailand Post Office had emailed it to me. For future applicants, it would be helpful if the Thailand post office provided the tracking number via email when purchasing prepaid delivery, rather than just a THP receipt.
Thank you all; this thread can now be closed.