I have 500MB data. I'm careful about not overusing it, as I'm usually connected to WiFi. In the last bill I found that I was charged a total of $25 in data for "Application Store Services". What does that exactly mean?
If you are seeing the "Application Store Services" on your phone, then it would be the app store.
As you said you are usually connected to wireless, if you have upgraded to iOS 9, go to Settings, Cellular and scroll to the bottom. If you see "Wi-Fi Assist" is turned on (Which it is by default), turn it off. This setting makes it so whenever the iPhone believes the wireless network is not strong enough, it will use your cellular connection to supplement it.
@Brian. To check for purchases made, login to your iTunes Store account. Click the Account tab and select Purchased. To the upper-right, you will see categories such as; Music, Movies, TV Shows, Apps, Books and Audio Books. Click on any of the categories to see if a purchase was made.