Why are there two transactions for different amounts showing in my account?

At the time of purchase you will see two separate amounts:

1. Your first instalment, which is charged to your credit card; and 2. The reserve (pre-authorisation) for the remaining balance of your purchase. This is a temporary hold against your available credit - it is not a charge and is completely interest-free.

Together these can look like the full purchase amount, but you have only been charged the first instalment.