When you approach an NPC porter on foot, wait until they stop to look at you. If one of them takes one of their crates and holds it out, drop one of your own crates next to them and take the one they were handing out. They will give you the crate, after which they will see the crate you dropped and pick it up. After that you'll unlock the achievement.
The crate they offer is randomised, and what you drop can be anything. I used hematic grenades, but I bet a blood bag or something works just as well.
If you have trouble finding porters, you can go increase your reputation with the Veteran Porter prepper, they'll give you
.