Episode #3.2 - S3-E2
Stupidity: Throughout the whole episode, no effort at all is made to find out where were the chocolates purchased, and the bottle of champagne that accompanied the box(es) is completely ignored, not even addressed in a throwaway line (such as that it's too common to trace, or whatnot). It's not directly part of the murder trick so it not even considered. The explanation of why the other incriminating box is still there is also quite ridiculous, since the murderer is left alone for enough time to do strike again with a very timely effort involving her leaving the only room of her bungalow (which involves her, the most known and conspicuous person on set, stalk her in plain view for an undisclosed amount of time), but could not find a few seconds to take a walk a few feet away and bury the box in the sand or bushes, flush the poisoned chocolates down a toilet, throw them at sea, just about anything. It's a bit of chocolate and cardboard, not exactly hard to dispose of that in a big set in the wild.
Episode #3.2 - S3-E2
Stupidity: Spoiler - In the flashback, apparently the killer 'framed' Susie by going as herself and on the phone going "Helloo, Susie speaking" next to the one guy that is a police informant - not the guy who sold her the fish poison. It would have made sense if she did that while she was buying the tetrodotoxin, but there's no way she could have known that that particular guy was going to talk to the police and they wouldn't be able to instead get a description from the original poison seller (that the police seems to ignore anyway and just let him deal unscrupulously - nobody has a good reason to own a bag of venom - in deadly substances).
Episode #3.2 - S3-E2
Stupidity: The whole case could and should have been solved in 5 minutes if only the cops did what any normal cop would have done; once it became clear (very early in the case) which rare poison was used; trace where it came from. Which in this episode is depicted as being ridiculously easy (all it takes is send Dwayne to the docks to give 20 bucks to his buddy), and is instead resolved very late because it is relegated to a little side-quest Fidel has to deal ineffectively with while the main characters psychoanalyse people and air their dirty laundry.
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.