Help with Bool Values? [UNANSWERED]

So I am making a Tycoon and I am adding perks that adds stats like Cash, Health, and Speed but I am having a problem. My Cash Dev product script works fine that isn't the problem its the one that adds speed. So when it is bought it gives you the selected amount of speed and when you die it is gone which is how I wanted it. But when I rejoined I quickly noticed that you get all of the speed that you payed for back which is practically really frustrating considering I CANT fix it at all.People have told me to use Bool value but in that aspect I am very lost and dont get it. Another guy said that the game is thinking that they are buying it again so it gives them the speed again and to add a Bool value. I ask them how I would do this but they explain it and I dont get it. It seems to short and not well said on how to do it... I am in deer need for a solution! Please help if you can!

Here is my script for the speed just in case it is needed:

local developerProduct = 20433249 local speedInc = 15

Game:GetService("MarketplaceService").ProcessReceipt = function(receipt)
    if receipt.ProductId == developerProduct then 
        for i,v in pairs(game.Players:GetPlayers()) do 
            if v.userId == receipt.PlayerId then 
                if v.Character and v.Character:FindFirstChild("Humanoid") then
                    v.Character.Humanoid.WalkSpeed = v.Character.Humanoid.WalkSpeed + speedInc
                end
            end
        end
    end
end