You can see the final price on the screen before you confirm the payment, as well as on the receipt from the application store. In some regions, the price also includes taxes. Your region is determined automatically based on the country selected in your application store profile.
(iOS) Sometimes several purchases get merged into one transaction on the receipt you get from the store. The receipt might also take several days to be sent to you, which is why you might get the impression that all of the purchases on a receipt were made on the day you got it. You can learn more about the topic in this article.
If you still think you got charged extra, please contact your application store's customer support. They'll be able to give you more details on your purchases and the tax policy for online purchases in your region.