How would I do a Developer product?

Okay, so I have a script that will allow you to purchase an in game item, but I'm stuck on how to have the item rewarded to you. I'm awarding cash with this one, how would I do it?

productId = 24209543

local MarketplaceService = game:GetService("MarketplaceService")
function UsernameFromID(ID)
    if type(ID) ~= "number" then
    return
    end
    local sets = game:service("InsertService"):GetUserSets(ID)
    for k, v in next, sets do
        if v.Name == "My Models" then
            return v.CreatorName
        end
    end
end
function giveRewards(player)

    return Enum.ProductPurcaseDecision.PurchaseGranted
end
MarketplaceService.ProcessReceipt = function(receiptInfo)
    giveRewards(UsernameFromID(receiptInfo.PlayerId))
end
script.Parent.MouseButton1Down:connect(function()
    game:GetService("MarketplaceService"):PromptProductPurchase(script.Parent.Parent.Parent.Parent, productId)
end)