The $4 for the empty plug-in is dead (at least for now), but the $4 back is still alive for refills costing $7 for a 2-pack. Not free but I did it because I already have 4 of these plug-ins without anything to put into them
(I also have a whole bunch of Febreze refills from the $100 of $100 Amazon Fresh deal a while back. Didn't realize they were just refills)