Let's just clarify 'How do we write down. . .' and 'How do we say. . .'
We write 'USD18,634.69(note the comma)' if we need to stipulate that we are referring especially to US dollars rather than Australian, Canadian or any of the many other dollar currencies; otherwise use $ before the amount.
We say 'Eighteen thousand, six hundred and thirty-four (US) dollars and sixty-nine cents.' (AE speakers commonly omit the first and.)