That depends on where you bought your subscription:
Google Play Store
This is how you request the invoice from Google: https://support.google.com/paymentscenter/answer/7520537?hl=en
Apple App Store
Please contact Apple support directly at https://getsupport.apple.com
FastSpring (7Mind Homepage)
You will receive an email from FastSpring with all the details of your subscription as soon as the purchase is completed. This email contains a link to FastSpring where you can manage your subscription. If you no longer have this email, send a message to firstname.lastname@example.org with your 7Mind profile email address so that we can request the invoice for you.