I believe you have to use a two step procedure

1. Update existing values:

IF EXISTS(SELECT p.ProductName,p.StoreName FROM Products p, temptable t

WHERE p.ProductName = t.productname AND p.storename = t.storename)

BEGIN

UPDATE Dbo.Products

SET Price = t.price

FROM Dbo.Products AS p

JOIN Dbo.Temptable AS t

ON p.ProductName = t.ProductName

AND p.storename = t.storename

END

2. Add items where the combination of productname and storename are not in the products table

INSERT INTO products (productname, storename, price)

SELECT t.ProductName,t.storename,t.price

FROM temptable t

LEFT OUTER JOIN products p

ON p.productname = t.productname AND p.storename = t.storename

WHERE p.productname IS NULL OR p.storename IS NULL

I hope this helps.