public override string ToString()
{
return($"Id = {Id} TransactionTime = {TransactionTime}{Environment.NewLine}" +
$"TotalPrice = {TotalPrice}{Environment.NewLine}" +
$"{string.Join<string>(Environment.NewLine, ShoppingItems.Select(s => s.ToString()))}");
}
private void ComputeTotalCost()
{
TotalCost = ShoppingItems.Select(item => item.Cost).Sum();
}