A disguised frumentarius will appear and inform the player character that the Legion will put some gear into boxes every few days, as long as the Courier does not become an enemy of the Legion. A warning is given that any further crimes perpetrated against the Legion in the future will cease the deal.
The boxes are located just as they are as a reward for Caesar's Favor, almost due southwest from the sniper's nest on a ledge in the mountain. Note that companions may have a hard time getting up the mountains or descending them, so it's best to ask them to wait somewhere more accessible beforehand.
PC The frumentarius may never appear in the game at all.
PC A possible fix for this bug may be to use the console command player.placeleveledactoratme 15d42f. This will clone the frumentarius and place him where you are, allowing you to talk to him.
PC Other possible fix for this bug may be to use the console commands prid 164ed0, disable, enable, resurrect, moveto player and player.moveto 164ed0, in that order. This will revive the frumentarius and place him where you are, allowing you to talk to him.