From a security engineering risk I don't think that would be an issue, because the same mechanism that catches malicious senders would be at play, the sender would have to identify through the app, with a payment provider and to the courier to send a package. The fake courier would have to sign up as a fake sender and be risk-exposed through the sender role.

Courier immunity does not confer much advantage compared to just signing up and having someone else send it. Except, it's true, that a trafficker could play both roles and self serve to avoid courier inspection/risk, there's some implications there for sure, but same as any job right? Pizza delivery guy could be selling drugs. It's not like transport is a niche job that might warrant specialized training and certification, it's like half the economy, tell me a commodity more central to business than oil, it can happen yes, but it isn't the end of the world if it happens on your business it's part of the trade, as long as you can deal with it, comply with the investigation, put preventive measures, and design the system with that in mind, I think it'd be ok.

Obligatory disclaimer. I am not a lawyer, this is not legal advice, just my personsal opinion on the matter