Poor people trying to make an easy buck aren't a problem. The problem is when they do so by means of participating in (organised) crime.
The problem is the organized crime. There's no end of poor people trying to make an easy buck, and arresting an endless stream of them for delivering packages is not productive, you will never eliminate the tools of such organizations, at least without devolving into despotism. You need to strike at the head, or take away their means of profit (by protecting victims, for example).
As to not knowing what they are a part of... seriously? In what conceivable Universe is someone that dumb that they believe that there is a legitimate reason why someone would want them to:
I didn't say that. I said they have plausible deniability, which is generally understood to mean they know what they are doing but evidence to prove it doesn't exist. They don't open the packages. They aren't doing anything illegal aside from under reporting income. Ergo, arresting them probably doesn't even lead to charges, let alone convictions. And even if it did, the person you caught is a disposable tool of the organization and will be easily replaced with another of the aforementioned endless stream.
No, they know very well that they are a part of something illegal and that at some point of the chain is someone being made to suffer. I'm not saying that they should be executed for said service, but at the very least a few months in prison or a hefty fine should be applied.
Sure, but you will not solve this by arresting yet more poor people and throwing them in prison. Do you want solutions, or vengeance? Arresting low-level couriers isn't, and has never been, a solution to organized crime.
And no, the argument "I'm poor so it's okay for me to steal from grannies." is not valid.
Of course it's not, but you do get a lot fewer thieves when fewer people are poor. It doesn't make it okay, but if you care about reducing crime and not simply punishing criminals after the crime has occurred, it is a huge factor.