Items on sale directly from Avast are only available at the time of the sale and it is a new subscription/registration which is active from the time of sale
Note I don't work for Avast buy am an Avast User like yourself.
This question has been asked before in the forums, someone wanted to take advantage of a sale price.
You could try - Submit a support ticket
https://support.avast.com/en-ww/contact/sales_other_general#pc -
Refunds, cancellations, billing, payment or license issues and click Next.
I need help with a different subscription or billing issue.
Here you could see if this is possible to extend the existing license.