The purchase shows on your iTunes account's purchase history : See your purchase history in the App Store or iTunes Store - Apple Support ? If not the 'charge' might just be : About payment card authorisation holds in the iTunes Store - Apple Support
If it does show does the app have an IAP restore option, or is there a FAQ/help section on the developer's website ? Have you contacted the developer ? If the developer can't or won't help contact iTunes Support from your purchase history or http://reportaproblem.apple.com or https://www.apple.com/emea/support/itunes/contact.html