Why can this script do both, the tix and robux, after I click it? (Nevermind)

I am making something where when a player clicks they can buy it. The items they are buying is actually from other players, because one it is to advertise and help them out, and two, I am not even in BC or Club :D. Anyway, I found a script where when I click the item, it will pop up saying like are you sure you want to buy it. Then when you click buy, then it means you bought it. But what makes me confuse is that, it doesn't include the tix with it, but only robux. This is the script:

local Id = script.Parent.Parent.ItemID.Value

script.Parent.ClickDetector.MouseClick:connect(function(player)
    game:GetService("MarketplaceService"):PromptPurchase(player, Id)
end)

Hope you know what I mean.