Solometer

Hi, I just downloaded affinity v1.4 from the app store . The price listed was at a 20% discount in Australia which comes to $62.99. The issue is that I didn't get charged for the app ? It downloaded and it opens. I have not played around yet, but I am concerned. How do I pay as I don't some message in a few weeks time saying I need to pay and the 20% is no longer available ? Thanks in advance, Solo