If you've bought a subscription in a mobile app, you'll receive an invoice either from Apple App Store or from Google Play Store.
Take a look at your Apple / Google account or contact the respective support to receive an invoice. For a Google Play invoice click here.
If you've bought a subscription directly from Billdu (on the website, via an email or discount link), an invoice will be sent to your email automatically after the payment is received. You can also download the invoice in your account on the website, navigate to Settings - Subscription - Orders and invoices.