How to toughen cookies
I'd imagine these are being stored in a simple glass / plastic display similar to what you'd see at a sari-sari store. 

Shalryn's suggestion is good, but it compromises the quality of the cookie a bit. I'd look at using some sort of desiccant first, and a simple electric fan behind the display to move the air around in order to take advantage of it. Humidity and lack of air movement is definitely a big problem here, and if you can find a way to control that, you might not have to alter the recipe (which sounds positively delicious and I'm going to try it).

Some rice flour in a small shallow tin might just be enough to do it. You'd have to experiment a bit, and ultimately you might have to alter your recipe, but those cookies are surely a hit and I'd hate to see you have to make them harder for logistic's sake :)  

If they're being sold in boxes, just add a silica gel pack and let folks know that they can't hit direct sunlight for too long. They are incredibly cheap (they run around 1.25 pesos from anywhere between 2-3 gram packets).