Whatever you do don't do it too soon or you are effectively paying twice, possibly worst leave it too late and the autorenew will kick in.
I have never used Digital River, so I can't speak from personal experience, but in the time that I have been on the Avast Forum as a voluntary helper.
To my recollection I haven't seen much related to Digital River. Presumably you are able to go into your Digital River account and change your option not to Autorenew ?
The renewal reminder from Digital River I guess should also give the option to cancel as this is a pretty standard requirement I would have thought.
As I said I use Avast Antivirus free version, but were I to be using a paid version of Avast I would follow this advice, I have never setup a renewable mandate for software, I prefer to manage that myself. Be assured the program will remind (nag) you when your subscription renewal is close, you may even get a discount offer to renew.
I did purchase two lifetime licenses for software in the past one is still going strong the other (for a firewall) ceased trading, but I still feel I got a good deal having used it for many years.