On the invoice appears a “Product Serial Number”. What is it ? Could it be retrieved somewhere ?
First, I’m an Avast user and not an Avast Team member.
Was this an email that you received after Purchase, if so it should have an Activation Code (not sure if this is what you are talking about).
That I believe is a reference number of your subscription, you should be able to enter that within your Avast Program.
Right click on the Avast system tray icon > Registration information, that should take you to the relevant area to either check or enter your Activation Code/Subscription.