From this:

// Changing amount                    
var position = item.position;
if (item.amount > 1)
{
    item.amount--;   
    item.MarkDirty();
}
else
{
    item.Remove();
}

to:

// Changing amount                    
var position = item.position;
if (item.amount > AMOUNTfromCFG)
{
    item.amount -= AMOUNTfromCFG;   
    item.MarkDirty();
}
else
{
    item.Remove();
}