I’ve been using your free Antivirus and just recently received an update for 2017 charging me $79. Why would I be charge $79 for a free service?
Likely because you have used a paid version in the past en did not end the subscription or you have used the 60 day trial en did not stop it in time.
But only avast can tell.
Ask them (ticket) > https://support.avast.com/support/tickets/new