Zombie Derby 2

To purchase a sticker, you will need to head into the garage and access the cosmetics menu by clicking on the paintbrush icon. Once in this menu, simply purchase a sticker which will cost you 250 Coins.

These stickers give you no benefit to gameplay, so I would recommend you fully upgrade your vehicle first so you can continue to make it further and further into Level 1 to earn more coins quickly. Once you fully upgrade your vehicle, you will unlock this achievement:

Here is a 100% achievement walkthrough for those that may be interested. This achievement pops around [09:30]:
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