See Get Apple apps for free on your iPhone, iPad, and iPod touch - Apple Support. It seems that the current layout of the app store doesn't quite agree with the description given in the article. I can only suggest that you persevere with the purchase of an app in that collection that you actually want to use. Either it will turn out to be free after all, or you can contact the iTunes Store team to report a problem and ask for a refund if you are charged.

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