How to properly use Developer Products?

I've been having some trouble with this over time. My problem is that someone could press Cancel (On the GUI Prompt to purchase the Developer Product) and 5 minutes later, they'd get the product. On other occasions, people would properly purchase the Developer Product, but they wouldn't receive an item. They would need to purchase it again. If I test the script in a server by myself, it would appear to work without any flaws. Does anybody have any pointers? Any help in advance is appreciated.

local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 21341246

game.Players.PlayerAdded:connect(function()
	print("start")
local DeveloperProducts = game:GetService("MarketplaceService"):GetDeveloperProductsAsync():GetCurrentPage()
for _, DevProductContainer in pairs(DeveloperProducts) do
    for Field, Value in pairs(DevProductContainer) do
        print(Field .. ": " .. Value)
    end
    print(" ")
end
print("end")
end)

MarketplaceService.ProcessReceipt = function(receiptInfo) 
	for i, player in ipairs(game.Players:GetChildren()) do
		if player.userId == receiptInfo.PlayerId then
			if receiptInfo.ProductId == productId then
				for i,v in pairs(player.Order:GetChildren()) do
					if v:IsA("BoolValue") then
						game.Lighting[v.Name]:clone().Parent = player.Backpack
					end
				end
				player.Order:ClearAllChildren()
				player.Order.Value = false
			end
		end
	end	
	
	local playerProductKey = "p_" .. receiptInfo.PlayerId .. "_p_" .. receiptInfo.PurchaseId
	ds:IncrementAsync(playerProductKey, 1)	
	
	return Enum.ProductPurchaseDecision.PurchaseGranted		
end

is it a local script?