All iOS apps are purchased from Apple on the App Store. Since your transaction takes place between you (the customer) and Apple (the retailer), you must obtain an invoice or purchase receipt from Apple.
Typically, Apple with email a billing receipt to your Apple ID email address when your payment method is charged. This may be a number of days after you have installed an app.
Find Your iTunes Store Purchase History
Please use this support documentation to see your purchase history in the iTunes Store on a Mac or PC: https://support.apple.com/en-gb/HT204088