When you make a purchase through iTunes, a confirmation email with your order information is sent to the email associated with the Apple ID you used. You can also find your order information by following these instructions on how to view your iTunes purchase history: http://support.apple.com/kb/HT2727 

If you need to contact Support for a purchase issue, please attach a screenshot of your receipt and make sure that the time and date of purchase is visible.

You can find instructions on how to take a screenshot here: https://support.apple.com/en-us/HT200289