If a tank (any tank) has only a single crack in it, it's almost definitely structural and/or impact/stress-related. Cracking caused by juices do not always manifest as hazing or a spiderweb but there will most likely be a few cracks.
As far as receiving one already damaged, I'm pretty sure Pop checks them before packing and I highly doubt that it left the shop in that state. On the other hand, I've heard of plenty of cases of people receiving pyrex tanks that have been crushed or shattered so it wouldn't surprise me at all if some of these arrive damaged. I hope you dropped a line on their website, or sent an email, when you got it--I know they'd want the opportunity to make it right...
I'm ok with using as-is. It seems once the crack is there, it relieves all the stress and doesn't continue. I agree with the potential shipping damage- I've gotten squished cartos in those little bubble envelopes. Something to consider if not already done--- ship with the screw Not screwed into the tank.
I'm still a happy customer- these tanks changed my vaping life. I used to fill 7 cartos 3x a day, and with this tank, I just squirt it full in the morning- and I'm done for the day.