If you have bought your subscription via our website, you will receive an email from us with the invoice in the appendix.
If you have bought your subscription via Apple (e.g. with an iPhone), you can go to the website https://reportaproblem.apple.com/ Go and download your invoice there.
If you have bought your subscription via Google (e.g. with an Android smartphone), you can use this help page https://support.google.com/store/answer/13714320?hl=de download your invoice.