UsingLinq.cs

var salesOfCustomers = source.GroupBy(x => x.Customer);

foreach (var salesOfCustomer in salesOfCustomers)
{
  var salesOfProducts = salesOfCustomer.GroupBy(x => (x.Product));
  foreach (var salesOfProduct in salesOfProducts)
  {
    foreach (var sales in salesOfProduct)
    {
      Console.WriteLine(sales);
    }
    Console.WriteLine("小計 P:{0}, {1}, {2}", salesOfProduct.Key, salesOfProduct.Sum(x => x.Quantity),
      salesOfProduct.Sum(x => x.UnitPrice * x.Quantity));
  }
  Console.WriteLine("大計 C:{0}, {1}, {2}", salesOfCustomer.Key, salesOfCustomer.Sum(x => x.Quantity),
    salesOfCustomer.Sum(x => x.UnitPrice * x.Quantity));
}
Console.WriteLine("総計 {0}, {1}", source.Sum(x => x.Quantity), source.Sum(x => x.UnitPrice * x.Quantity));