Well I just received my Mastercard statement with one of these charges, in this case GBP14.99, for which I have no recollection of ordering. I have been using Avast free on my pc for years without such a problem, and on my mobiles for two or three years.
I went to the Digital River link supplied here and found a special place to enquire about such charges:
https://www.findmyorder.com/sstore?Action=DisplayHomePage&SiteID=findmyor&Locale=en_USIt is described as follows:
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Important
If you have a charge on your recent billing statement with a description such as, "DRI*" "DRI" or "Digital River" and do not recognize this charge, please look up your order using your email address and last 5 digits of your credit card.
If you are unable to find your order using your email address and last 5 digits of your credit card, please look up your order using your order number and password. Forgot your password? Click here.
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Unfortunately the first option did not come up with a response and the second was not applicable since I did not have an order number or password. But this is obviously a problem affecting many people recognised by DR sufficiently for them to have this special page on their site.
I passed the message to their CS department through their Contact system and am awaiting their response.
I'll post it when received.