I dropped $150 on school supplies for 3 kids, kindergarten, 3rd grade, and 7th grade. $400 in my opinion is definitely excessive.
Yes, you should confront her on it like a mature adult. That's what you would do if the mechanic told you he found some things that needed done on the car so he went ahead and did them and just added to cost to the bill. Yes, the kids needed school supplies, but, her spending was out of line and I can almost guarantee that she didn't honestly spend that much on supplies. You could tell her that you thought she was excessive and you'll gladly split the cost with her when you see the receipt as $400 seems excessive. Maybe she bought clothes too.
OK, that seems somewhat confrontational, but, you do need to set a boundary there and find a way of dealing with it otherwise there is a good chance that she will take advantage of you again in a similar vein.
