I experienced exactly the same issue. But when I checked my paypal there is not a £0.00 transaction record from avast or for that date.
I received the same response from avast in reg to ID number of purchase. If they charged me after 30 days I will dispute through paypal and will let the ASA and CAP agencies know so they can open an investigation on trading standard practices. Quite bad there is not costumer service offered when they are selling a product, I believe it goes again the consumer protection act.