""Unable to download the app at this time" but Apple charged me anyway

I tried to buy an app through the app store last Saturday and it kept saying ""Unable to download the app at this time". I turned my iPad off several times and tried again but it still didn't work. I downloaded another app and it did work. This morning I saw that Apple still charged me even if the app couldn't be downloaded. And since I tried several times, I guess I will be charged several times as well. I stop trying since that day so please help me download it or refund me.