Relief packages are denoted by a blue hexagon with a downward facing arrow. The controller will begin to rumble a lot as the plane that drops them flies in. If you reach the package fast enough, there will be two boxes, and I believe both of them count.
If you take a little bit to get there, several of Rais's men will be there, having already looted one of the boxes.
Take too long and it will disappear.
I believe sleeping will force a new plane to come in and drop a package but it could be coincidence.
There are also pre-spawned supply drops, which are marked with a white hexagon instead of a blue one. There are only a few in the first area but there are many in the second area. Although the ones in the second area belong to Rais, Ehrenfriend confirms they do count.