Check-in explicitly asks you "have you packed your bags yourself", and then you have to either say "no I have this random package from a stranger which might contain anything" or lie to customs.

TBH, I can't really think of a market for this that isn't contraband. The "last mile" looks really annoying as well.

Edit: I think it's a legit marketing question for OP. Name three different kinds of item someone might want to use this service for.

I'll even give you one: there's already a small cottage industry of reshipping companies from e.g. Japan, who will let you buy stuff from companies that won't themselves do international shipping. Ship to re-shipper, who then handles the international part.

You might be able to get a market started if your model starts with only items bought from legitimate retailers. Effectively a really long distance doordash.

The only way in which this makes sense in international is something like courier flights. I don't know if this even really exists in the form it used to, but there used to be small-scale services that would buy regular airline tickets (in advance but transferable) and then resell the seat but not the luggage allowance for major city pairs like NY-London. A person who can fly on short notice with no baggage allowance gets a cheap ticket, and the customers get their essential items delivered more cheaply than last-second air freight.

A key element is that the passenger was contracting out their baggage allowance and so didn't ever interact with the items - they never even saw it. So no liability.

I think it probably works fine for national delivery couriers filling space in their van with additional extra bulky items; services to disintermediate them to move heavy goods for less cost than a dedicated courier already exist and some of them even wrap suitable insurance around it.

Internationally if it's P2P rather than P2companythatdoesthecustomspaperwork it's pretty much pure smuggling-as-a-service, and yes, people who kindly help carry the stranger's Colombian souvenir on their passenger flight for a small fraction of the ticket cost will find themselves being jailed at the other end.

I can easily imagine a market for this, because I was in the market for this until last week.

I had a large, bulky, and fragile package I needed to send to Florida from New Jersey. The shipping corps were happy to do it for me for $500+, and no guarantee that it wouldn't arrive as a box of shattered glass.

I ended up finding someone in town who happened to be driving there, and was kind enough to deliver it for me. They still offer no such guarantee, but they also were kind enough not to charge me for this!