Even though Costco hasn't indicated material cost variances from its suppliers, a deflationary environment forces it (and other grocery peers) to compete with the grocers that are already passing on savings to customers via discounted stock-keeping units. Consider Costco Wholesale (NASDAQ: COST) shares, which pulled back nearly 9% last Friday following the company's fiscal first-quarter 2019 earnings report. It's rare for Costco stock to move so dramatically in a single session.