If you want to register Calculation over the internet, you may be afraid of Credit Card Fraud. However, there is a simple solution to this:
Get a debit-card account that does not allow any credit. (Note: If you are 17 or over, you can get such an account at the Halifax.)
Only keep a small amount of money in it at a time.
Now you can order things over the internet however much you want without any fear!