Mine was posted one week before the due date and I still have the post office receipt so I think the best thing to do is wait for the acceptance or rejection. I notice that the immigration website has recently been updated to make this rule clearer.
Is there an office closer to Bangkok that's giving out these extensions to a tourist visa without an embassy letter up to 26 October? For those in Bangkok a trip up to Pai is nice but somewhere closer would be a lot more convenient.