How do you give a player who buys a developer product an item?

I've went through the same pain you did. Basically you look for the player then check what product the player bought then add the item. What I do for PvP Battle. All you have to do is change what is in the if statement for the product id checker.

local prodId = 1
MarketplaceService.ProcessReceipt = function(receiptInfo) 
    -- find the player based on the PlayerId in receiptInfo
    for i, player in ipairs(game.Players:GetChildren()) do
        if player.userId == receiptInfo.PlayerId then
            -- check which product was purchased (required, otherwise you'll award the wrong items if you're using more than one developer product)
            if(tonumber(receiptInfo.ProductId) == prodId)then
                --TODO: Buy the Ocherous Katana of the Setting Sun
                game.InsertService:LoadAsset(25545089).OrangeKatana.Parent = player.Backpack
                game.InsertService:LoadAsset(25545089).OrangeKatana.Parent = player.StarterGear
            end
        end 
    end
    -- tell ROBLOX that we have successfully handled the transaction (required)
    return Enum.ProductPurchaseDecision.PurchaseGranted     
end

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