The embassy website only mentions financial proof equal to the standard 20k baht for all visa applications. They state that is $700 on the page you posted a link to.
Are you sure that you do did not apply for a Non-OA visa by mistake when doing the online application, It does require proof of 65k baht income.
Not sure it is worth the effort to apply for a non-o visa for only a 90 day stay. A tourist visa would allow a total stay of 90 days by extending the 60 days it allows for 30 days at immigration.