To clarify the situation and correct DavidR’s statement, I’ve been renewing premium security for about ten years now.
Is going through Digital River really the only way to renew my subscription?
yes I think, this is the avast regular way.
Every time, the billing comes from Digital River when you use your avast account or the avast store, and they’re the main (only?) partner if you use avast, at least for European users.
nb: more than 10 years ago, I used to go through Nexway, but it was dealt with separately on the Nexway site.
With Digital River, I’ve never used their site, but it’s handled directly in our avast account.The order number that avast asks for in all contacts is even indicated on the Digital River invoice.

I’ve never had any problems with my payments.
I think what makes the bad reviews on Digital River is that they charge for third-party products that use auto-renewals like avast, and people forget to disable these in the product account (here avast account) and get the impression of a scam when they end up with a Digital River charge they don’t know about.
As far as the purchase date is concerned, which according to DavidR is the starting date for using the product, this is generally not the case.
If the e-mail addresses of the initial purchase and the renewal are the same, avast proceeds with continuity, one-year extension on expiry of the previous one and not 2 subscriptions at the same time, I speak from experience.
However, if this is not the case (change of e-mail address in the meantime), you can contact avast support, who will merge the 2 subscriptions
This is what just happened to me, my previous subscription (automatically deactivated) expires on October 26, I accepted an interesting offer on October 12 using another email address (paypal), avast via CHAT merged the 2 and the expiration is now October 29 (I even gained 3 days). 