[@AlphaLemming]

I'am using this for CraftStore:

local function OnSlotAdded(bag,slot,data)
    < example > data.uid = Id64ToString(GetItemUniqueId(bag,slot))
    < your code >
end
 
local function OnSlotRemoved(bag,slot,data)
    < your code >
end
 
SHARED_INVENTORY:RegisterCallback('SlotAdded',OnSlotAdded)
SHARED_INVENTORY:RegisterCallback('SlotRemoved',OnSlotRemoved)

This functions are instantly called on every item change. You can also use 'SlotUpdated'.