The process used in creating this 'invisible ink' also produces its revealing-solution as a byproduct. Anything written in Courier's Ink is completely un-noticeable on the page, until it is dipped in the 'revealing-solution' at which time the ink appears. Revealed courier's ink is always red, and most people with sensitive information to hid usually write with normal ink on the page in another colour, to throw off suspicion.
A single dose of Courier's ink is sufficient to write 3 pages text. Courier's ink cannot be used to record a spell, as it is invisible even when writing with it, making it impossible to draw the precise symbols used in magic spells.