Help with pointshop speed boosts

I need some help with making speed boosts for my deathrun server on garrysmod.
I am using point shop by _undefined.

This is the current code that I have for my 2 speed boosts

speed boost 1


ITEM.Name = "Speed Upgrade 1"
ITEM.Enabled = true
ITEM.Description = "Gives you extra Speed!"
ITEM.Price = 5000
ITEM.Model = "models/weapons/w_toolgun.mdl"
ITEM.SingleUse = false

function ITEM:OnEquip(ply, modifications)
    PrintTable(modifications)
    ply:SetWalkSpeed(325)
    ply:SetRunSpeed(600)
end

function ITEM:OnHolster(ply)
    ply:SetWalkSpeed(225)
    ply:SetRunSpeed(500)
end


and speed boost 2


ITEM.Name = "Speed Upgrade 2"
ITEM.Enabled = true
ITEM.Description = "Gives you extra Speed again!"
ITEM.Price = 7500
ITEM.Model = "models/weapons/w_toolgun.mdl"
ITEM.SingleUse = false


function ITEM:OnEquip(ply, modifications)
    PrintTable(modifications)
    ply:SetWalkSpeed(425)
    ply:SetRunSpeed(700)
end

function ITEM:OnHolster(ply)
    ply:SetWalkSpeed(225)
    ply:SetRunSpeed(500)
end

It all seems to work but the only problem is that people can just buy the second speed boost and get maximum speed without having the buy the first one, so i was wondering if there was an code to allow me to just add an extra 100 speed to their running and walking speed, rather then the code I am using.

Would be awesome if anyone could help me out.

Are you using TTT or an other gamemode?