How can I request the invoice for my purchased subscription?

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 [e-mail] with your 7Mind profile email address so that we can request the invoice for you.