Perhaps you missed the OP...
"This is for Australian Retirees.
Seeing how the Australian Embassy does not provide proof of income, in this case the age pension, how do you prove proof of income for the "Marriage" Visa"
The OP is Oz . Posts re embassy letter for income letter are off topic.
Advice in future start your own thread to obtain best advice.
Entering Thailand with a non O and then subsequent extension using embassy letter would have been best advice for your situation.