By Dan Weil At March 31, Costco had 829 warehouse locations, up 2.7% from 807 a year earlier. The Action Alerts Plus team offered commentary Thursday concerning several of the stocks it follows. Retail titan Costco Wholesale (COST) – Get Costco Wholesale Corporation Report reported that comparable sales surged 12% in March from a year earlier, excluding the impact of gasoline prices and foreign exchange. Costco’s net sales rose 17% to $130 billion in the first 31 weeks of the company’s current fiscal year compared with a year ago. “Those figures confirm our thesis on COST shares, that staring …