How could i make this Bigger Duffel Bag Gamepass?

You'd use PlayerHasPass from the gamepass service. https://wiki.roblox.com/index.php?title=API:Class/GamePassService/PlayerHasPass

It would look something like this.

Players[i].Robbing.Value = true
if game:GetService("GamePassService"):PlayerHasPass(Players[i], gamepassid) then -- Replace "gamepassid" with whatever gamepass you're using. In your case it would be 1607774359.
    if Players[i].CashCollected.Value < 3500 then
        Players[i].CashCollected.Value = Players[i].CashCollected.Value + 10
else
    if Players[i].CashCollected.Value < 1000 then
        Players[i].CashCollected.Value = Players[i].CashCollected.Value + 10
        end
    end
end

I haven't tested this however and it may not work. I'm not too sure but you can sort of get an idea of how it should work.

Also I would strongly suggest storing important values like Money and such, serverside using remote events and functions with filtering enabled so that you can avoid exploiters. https://wiki.roblox.com/index.php?title=Remote_Functions_%26_Events

Edit:

Try this?

local debounce = false
local playerfound = false

function CreateRegion3FromPart(Part)
    return Region3.new(Part.Position-(Part.Size/2),Part.Position+(Part.Size/2))
end

function GetPlayersInPart(part)
    local region = CreateRegion3FromPart(part)
    local partsInRegion = workspace:FindPartsInRegion3(region,nil,math.huge)
    local Players = {}


    for i,Part in pairs(partsInRegion) do
        local player = game.Players:GetPlayerFromCharacter(Part.Parent)
        if player then

            for i, v in pairs(Players) do
                if Players[i].Name == player.Name then
                    playerfound = true
                end
            end 

            if playerfound == false then
                table.insert(Players,player)
            end
            playerfound = false
        end
    end

    for i, v in pairs(Players) do

        if Players[i].Robbing.Value == false then
            game.ReplicatedStorage.ShowRobberyGUI:FireClient(Players[i])
        end
    Players[i].Robbing.Value = true
    if not game:GetService("GamePassService"):PlayerHasPass(Players[i], 1607774359) then
        if Players[i].CashCollected.Value < 1000 then
            Players[i].CashCollected.Value = Players[i].CashCollected.Value + 10
    end
else
        if Players[i].CashCollected.Value < 3500 then
            Players[i].CashCollected.Value = Players[i].CashCollected.Value + 10
            end
        end
    end
    return Players
end

while wait(0.2) do
    GetPlayersInPart(game.Workspace.Detector)
end