As far as the postcard goes, I’m afraid there doesn’t seem to be much to it beyond what you can see in the picture I already posted. I haven’t really been paying attention to any of the Descent of the Durtle stuff, though, so maybe you can find something I’m missing?
Here’s the back:
And here’s the card held up to a bright light (much to the confusion of our dog, who watched me standing on a chair to take these):
I even tried heating it with a hair dryer 
, but alas, no hidden message appeared. That unfortunately exhausts all of my training in detecting invisible ink.
I’m ridiculously busy this week, but if you can think of something else you’d like me to try that’s (1) quick and easy (i.e. doesn’t require any special equipment or travel) and (2) won’t result in me burning my house down, I’m happy to do so.