All the in-app purchases are processed through the account you used to download the game.
In order to make a purchase using another account, you have to uninstall the game, log into the preferred Google Play account on all the devices you play Mystery Matters on, and then install the game again. Please follow these step-by-step instructions:
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Go to Settings and add the preferred account to your device (Settings → Google → Account → Add account)
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Add a preferred payment method for the newly-added account.
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Uninstall Mystery Matters from your device, then launch the Google Play Store and search for the game.
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Check the Account tab in the Google Play menu to make sure you've logged into the preferred account.
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Install Mystery Matters again, then open the game and make the purchase.
We also suggest that you read this article on how to add Google accounts to your device.