All information about your current subscription and active services can be found under your account when logging in via the Nord Account website. You will be able to see your billing history as well as get a receipt or an invoice for your purchase.
In order to do that, just go to the Billing history category and you will see all of your payments made for any of Nord subscriptions:
Then, just find the wanted payment and click on the Receipt or Invoice option depending on which one you wish:
It will be downloaded to your device as a .pdf format file.