UsingKeyBreak.cs

var oldCustomer = "";
var newCustomer = "";
var oldProduct = "";
var newProduct = "";

var totalQuantity = 0L;
var totalPrice = 0m;
var totalCustomerQuantity = 0L;
var totalCustomerPrice = 0m;
var totalProductQuantity = 0L;
var totalProductPrice = 0m;

foreach (var sales in source)
{
  if (oldCustomer == "")
  {
    oldCustomer = sales.Customer;
    oldProduct = sales.Product;
  }
  newCustomer = sales.Customer;
  newProduct = sales.Product;

  if (oldCustomer != newCustomer || oldProduct != newProduct)
  {
    Console.WriteLine("小計 P:{0}, {1}, {2}", oldProduct, totalProductQuantity, totalProductPrice);
    totalProductQuantity = 0L;
    totalProductPrice = 0m;

    oldProduct = newProduct;
  }

  if (oldCustomer != newCustomer)
  {
    Console.WriteLine("大計 P:{0}, {1}, {2}", oldCustomer, totalCustomerQuantity, totalCustomerPrice);
    totalCustomerQuantity = 0L;
    totalCustomerPrice = 0m;

    oldCustomer = newCustomer;
  }

  totalProductQuantity += sales.Quantity;
  totalProductPrice += sales.UnitPrice * sales.Quantity;
  totalCustomerQuantity += sales.Quantity;
  totalCustomerPrice += sales.UnitPrice * sales.Quantity;
  totalQuantity += sales.Quantity;
  totalPrice += sales.UnitPrice * sales.Quantity;

  Console.WriteLine(sales);
}
Console.WriteLine("小計 P:{0}, {1}, {2}", oldProduct, totalProductQuantity, totalProductPrice);
Console.WriteLine("大計 P:{0}, {1}, {2}", oldCustomer, totalCustomerQuantity, totalCustomerPrice);
Console.WriteLine("総計 {0}, {1}", totalQuantity, totalPrice);