Excluding the impact of deflation in gasoline prices and adverse foreign currency fluctuations, Costco's comps for May rose 6%, reflecting comparable sales growth of 5% in the U.S. and 8% at its international outlets. Costco Wholesale CorporationCOST reported flat comparable-store sales (comps) for the four weeks ended May 31, 2015. Costco's net sales for May came in at $8.98 billion, up 2% from $8.78 billion recorded in the comparable year-ago period.