Don't forget that you can price match at Walmart. Get all the other stores loss leader items in one place. You can also see if you can wait to buy lunchboxes until Target marks them down to 75% off. Near us, they get marked down by the second or third week of school. I used to buy a few extra lunch boxes when they did that. We've had lost lunchboxes or spills that ruin the lunchbox. It is nice to have a backup available.
Also, a few years ago I got smart and bought my girls LL Bean backpacks. They were more expensive than other brands (about double). But they are guaranteed for life. My girls have use them for 3 years now and they hold up great. But if they ever get a tear or a hole in them, LL Bean will replace them for free!!! Even though I spent more up front, I have ended up saving money because I haven't needed to replace the backpacks every year.
For us, the costly thing is the large 3 ring binders. The girls need them for school and they generally cost around $15 each. But they start breaking by the end of the first quarter and really need to be replaced at least once during the school year. I keep searching the ads to see if I can find them on sale but so far I haven't. In the past Staples has had buy 2, get one free sales on them but I haven't seen that yet this year.
The other big purchase for us was the TI84 calculator. I was really happy that I found a used bookstore near the community college where I bought one last May. I only paid $59 for it, which is a bargain compared to the $109 it is on sale for at Staples and other places. Last year when older dd was starting Algebra I bought one on Ebay for about $60 but had to pay shipping too. I wish they would let them use their itouch instead of insisting on the calculators. With the itouch you can get an app that does the same things for about $3.