When your order gets canceled, you will not be charged by the Play Store. Please note that it is possible, however, that the Google Play Store puts a "reservation" on your credit card, equal to the value of the intended purchase. This reservation is temporary only will clear automatically after a short while.
Your purchases are probably canceled if your billing address and/or the name registered in your Google Wallet account do not match the ones provided by your credit card company. In order to fix this, please contact your credit card issuer to confirm that your details are correct. Then, log into Google Wallet at wallet.google.com, choose "Payment Methods", and make sure that the information there is the same as the data stored by your bank.
If your purchases are still getting cancelled automatically, please go to https://support.google.com/googleplay/digital-content/?hl=en for further help concerning the Google Play Store and purchases on Android devices in general.