If you are trying to make "Airdrop's" a command to buy it won't work in the way that you are thinking. 

You can set the cords for it to drop for example at an NPC Shop location or a preset location instead. But for now,

I suggest using the game item supply.signal and adding it to the shop both as an Item and then put the item into a shop tab. 

"Supply Signal":{
   "buy":"200",
   "img":"https://i.imgur.com/BPM9UR4.png",
   "item":"supply.signal",
   "cooldown":"60",
   "sell":"0"
},


"Misc": {    <---- Shop tab example
      "buy": [
        "Supply Signal"   < Item display name we just made for it above ^
      ],
      "description": "You currently have {0} coins to spend ",
      "name": "Misc",
      "sell": []
    },

If you wanted to do the commands anyway

"cmd": ["supply.drop 625.4652 0.4005555 -1881.23" ],    <Preset location example

"cmd": ["supply.drop \"$player.x $player.y $player.z\""],  <drop on player? I think.. Haven't tested