The stuff they get issued is overwhelming. I think they get at least four kinds of shoes on top of the flip flops. Don't worry too much because everyone is going to look like the dickens within a few days with or without that last thing you forgot to send along. And they do share what comes in the boxes, so shop or bake for a group.
If you want to bring some smaller items like a lighter or cough drops you can stick them in one of the boxes for sale by the parents club. At R Day they offer a lineup of approved junk food and commonly needed items that you load up yourself, add a personal note and they deliver within a week or so.
Remember to use two gallon ziplock bags to group like items, wear a set of broken in running shoes, and no matter what you do something will be wrong and you'll get yelled at. That's just a goal for the cadre and your turn will come. Have fun.