I never bought any product and I stopped using Avast many months ago.
What Avast Product/s were you using ?
Were they trial products, you would have given your payment details and at the end of the trial period, you would have been changed. Otherwise how would they have got your details
If you weren't using trial products, when you purchased you should have received an email, that should have had your order ID.
Did you receive any email from Avast ?
The most common reason for non-receipt of emails are:
Intercepted by your email provider or email program and sent to the spam folder.
Making a typo in your email address.
Using an old email address that is no longer active.
If you haven't got an Order ID enter 00000000 and should get you past that hurdle and give you a chance to explain the circumstances. Give as much information as possible to identify this as you can (you can include the subscription code number) to help identify the item/s.