Answer
Memberships purchased via web version:
You can easily view and download your invoices by clicking your photo in the top right corner of your XING page (accessible via www.xing.com) followed by memberships & invoices.
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Memberships purchased via the Apple App Store:
If you purchased your membership via the Apple App Store, please contact the App Store directly for invoice modification or invoice delivery.
Memberships purchased via the Google Play Store:
If you purchased your membership via the Google Play Store, please contact Google support directly for invoice modification or invoice delivery.
You can also get your Google invoices online here