I do international mailings regularly and if it's a package of a certain size you must declare what's in it for customs. I get that people can lie, but it really doesn't make much sense to send drugs from the US to Turkey.
They do behind the scenes checking of packages in post offices, too. It's not so simple to just send drugs, which is why a courier is used.
I'm not sure why it's so unfathomable, but a lot of drugs originate from more lawless regions (not saying Turkey is among them, but there are some close by) and places where drugs are cheap. There's a demand for drugs in areas where they aren't as readily produced or where the strains aren't as potent. That's to say if any trafficking was happening that this was drugs, which I'm still not convinced of.
The package could contain a document, addresses, or other things that weren't meant to be found or traced by electronics or emails. I doubt it was something serious, because professionals would likely use the email draft system that is common among spies and those who don't want a paper or document trail. They write a draft email, the recipient gets it and responds through drafts deleting as they go.