An interesting thread. I never realized that Departure Tax Stamps even exists (although as suggested in the link provided, they are classified as a "revenue stamp").
When web searching for some countries that use "stamps" for such taxes, I was surprised to learn that Chicago, Illinois (USA) also implements a $4 "airport departure tax" imposed on cab/livery vehicles, that may be paid by a stamp (although the "stamp" is hardly a philatelic treasure) as shown here:

However, I suppose any "stamp" is collectible to a true philatelist!