To get the invoice, you need to go to the platform through which you are subscribed:

Google Play – https://support.google.com/store/answer/6160235?hl=en-IN
Apple – https://support.apple.com/en-us/HT204088

If you are subscribed via desktop, type in the email address you used for a subscription here and you will receive an email with a link to your account management – https://windy.onfastspring.com/account.