FWIW, there are quite a few apps that offer additional "in app" purchases; here is the relevant excerpt from the iTunes Store T&C - did the app you purchased not have a note that it offers those?
IN-APP PURCHASES
Certain App Store Products may include functionality that enables you to receive additional services, or licenses to additional functionality or content for use within the App Store Product ("In App Purchases"). In App Purchases that are consumed during the use of the App Store Product (for example, virtual ammunition) cannot be transferred among devices; can be downloaded only once; and after being downloaded, cannot be replaced. Once a consumable In App Purchase is acquired and received by you, Apple shall be without liability to you in the event of any loss, destruction, or damage. All In App Purchases are deemed App Store Products, and In App Purchases received within Third-Party Products are deemed Third-Party Products, and treated as such, for purposes of these terms and conditions.
You must authenticate to acquire In-App Purchases separately from any authentication to obtain App Store Products by entering your password when prompted, but once you have authenticated to obtain an In-App Purchase, you will be able to acquire additional In-App Purchases for fifteen minutes without re-entering your password. You can turn off the ability to acquire In-App Purchases on your iOS Device by following the steps outlined at http://support.apple.com/kb/HT4213.
from here (US version):
http://www.apple.com/legal/internet-services/itunes/us/terms.html#SALE