As an admin, you can find the invoices for your Papershift subscription online at any time via your account.
Click on the gear icon (Settings) in the lower-left corner, then on Subscriptions/Addons. Here you will now find the following section:
Click on "⇾ To the customer dashboard", there you can change your payment details, view the individual subscriptions, and download the invoices. Just click on the individual subscriptions and go to the download symbol on the right.