We only charge for a transaction fee when an order is matched. In other words when Australian Dollars are converted to bitcoin at that point in time.
If you used our easy to use Buy wizard we automatically convert your Australian Dollars into bitcoin at the best available rate. What this means is that your order may get split up into multiple smaller orders to get the best price available.
The total transaction fees you pay will still be the same if you had one trade at $5,000 or two and $2,500. The fees are calculated on each amount and can be checked within the transaction table under account settings.
Within the transaction history table you can click on any line item to get more details including a specific break down of fees paid.