I've been getting those demands for payment even after having paid!
I pay instantly (or as soon as I wake and have had my coffee, if it's something I won in the middle of the night).
I'm not annoyed at the seller, but that's because I know it isn't the seller sending those pushy emails. But I can well imagine people who don't know thinking, "Get off my back!", under the impression it is the seller sending those poorly worded and hectoring emails.