You will usually experience the 'Your payment failed or transaction has been canceled' error due to Connectivity Issues. So please be sure to check the Network connection and ensure that the connection is stable.
In case, you are prompted with the 'You already own this item' error message then the purchased item/credits will be added immediately to your account on reload.
Note: You will not be charged twice nor the items/credits will be purchased multiple times. For more details, please reach out to our Support Team.