1. Arch supports ... Dr. Scholl's are bigger than AirPlus so it depends for what you choose. Sometimes I even combine the two because I have high arches.
2. Sometimes under ball of foot supports, maybe even to make the heel a little less high-feeling. These help with rubbing, and if you need that help but not the padding, use the new Dr. Scholl's rub strips.
3. Under-heel supports unless there's sufficient padding there, usually there's not. With open shoes the massaging gel ones let my foot slip though.
4. Of course with most shoes I need those rub prevention strips.
5. Since I put a ton of this stuff in my shoes, I now wear a bigger size. Haha. So maybe size up if you follow my advice. Besides, sometimes my old size was tight or brands didn't offer half sizes. (Sometimes my feet swell a bit or more.)
I cannot go above 2.75" but that might be because I have kind of small feet. Used to be 2.25" though! Yay for now, haha.
Oh! Also - it matters where you walk. A mall is much better than a parking lot craft fair, for example.