//adds an invoice to an airline
public static void AddAirlineInvoice(Airline airline, DateTime date, Invoice.InvoiceType type, double amount)
{
if (airline.IsHuman && GameObject.GetInstance().HumanAirline == airline)
{
GameObject.GetInstance().AddHumanMoney(amount);
GameObject.GetInstance().HumanAirline.AddInvoice(new Invoice(date, type, amount), false);
}
else
airline.AddInvoice(new Invoice(date, type, amount));
}