Buyer pays £4.50 plus £3.39 = £7.89. The buyer psys for the postage, but does not buy it.
You still have to buy the postage, are you sure you have not duplicated buying the postage, if so you will have to cancel one.
All transactions show in PAYMENTS, which is where you should look. It should show you received £7.89 and what payments have been taken out.
In my opinion it is best to avoid getting involved in buying postage through ebay, do it yourself you can then ensure you get it right.