FWIW...I agree with the rest of the guys on here about buying the school supplies on your own & not giving her anymore $$$!
I also agree with the way that you worded your email to W about things. It's not rude, straight to the point, and carries a little bit of confidence without being arrogant!
If she gets/acts livid, then as my IC states: "That is for her to deal with & for you not to respond"! Many times has Starsky & Denver stated to me that they will continue to lash out at you for many reasons. Absorb it, acknowledge it, then $hit it out in the toilet later with the rest of the waste in your body