Your comments
I've examined particular instances of the issues closer and I believe the solution is that the filter is being applied before the title of the feed is prepended onto the mail subject. I was including the title of the feed in the filter, so obviously my approach failed.
Now, this makes the issue non-existent but that highlights some need into adding "begins with..."/"ends with.." markers (I was including the title so to make sure only "Deals" as the subject's first word is catched). Otherwise, sorry for the trouble.
@the blog entry announcing beta:
The plan is sound, and the new beta service is getting better by the day but I'd advise against making filters a premium service. After all we're talking e-mails here, whereas these days it's insanely easy to employ user-end filtering within e-mail clients, desktop or www-based (I for one have been using the insanely easy but powerful filters in gmail). In fact, you should be encouraging people to use filters on your side - the more they do, the less unnecessary e-mails go out, and the less bandwidth you use.
Customer support service by UserEcho