I wondered about that too but it was either follow the published instructions or fight the bureaucracy. Have heard the phrase "don't try to fight city hall" before.
I saw that too and it rejected my passport number as ID. Managed to bypass that part and pay the 100 Baht, but still not 100% sure it is acceptable to the "authorities". The rub is, why are Americans being sent to a foreign website, which they cannot normally read, to obtain passport services for Americans? I have renewed my passport and my kid's passport several times already, and this has been the worst regarding a simple payment. 100 Baht... I'd be very happy if they added this $3 to the renewal fee.
I wanted to share some of the travesties I found while trying to renew my passport so that others may be aware. You will have your own experience and hopefully will be a happy happy one. Good luck.