This is the best option. It’s very unlikely it’ll affect the backpack, it degrades the adhesive, and then evaporates. Pull it off when it’s still wet. If there’s still residue after the label is off, use a paper towel with alcohol on it to rub off the rest. Might take a few passes.

(What’s weird is the Walmart stickers like this I’ve received have post-it note type adhesive, not permanent adhesive.)