Backpacks, pencil cases, water bottles, and indoor shoes rarely all need replacing; check what can be cleaned or repaired first.
A back-to-school supply list isn't really a shopping list for buying everything new; it's a checklist for confirming that what you already own is still safe and comfortable to use next term. Look first at the parts that look fine on the surface but are easy to overlook, like a backpack zipper, a water bottle seal, or how well indoor shoes still fit, and you'll likely find more items than expected that just need a wash or a small repair.
The most reliable guide is always the latest supply list sent home by the school, since requirements shift by grade, school, and subject, so don't just copy last term's list or a friend's shopping cart. Have your child pull each item out, try it on, and test that it works, then sort everything into three piles: already have it, needs repair, or needs to be bought.