According to our Terms and Conditions (https://www.zeptolab.com/terms), all purchases in the game are final and can't be refunded. You are responsible for the purchases made on your device by you, your family and friends.
Bullet Echo is a multiplayer title where people compete and interact with each other. Refunding money spent by accident will provide you with something for free, which is unfair to other players who were working hard to gain the same resources or upgrades.
You can find detailed instructions on how to prevent accidental/unauthorized in-app purchases here:
Android: https://support.google.com/googleplay/answer/1626831
Android: https://support.google.com/googleplay/answer/1626831