The second edition (hardbound with a white cover) of Feenstra's Crete handbook was published by collectio
I don't think it's still in print but I do recommend it strongly.
It's a very useful reference for many aspects of Cretan philately.
It's clear the postmark is scarce and collectable but how much would a collector pay for the postmark alone?
I've just checked the catalogue of collectio's auction #48 which featured Rienk Feenstra's own collection and was held in 2005.
This auction appears to have included a copy of this postmark on a reduced envelope, described as "the only known strike of this handstamp" (!) with a starting price of €300.
Unfortunately, I don't have to hand the prices realised from the sale so I don't know if this lot sold or if so for how much.
Good luck with this!