Costco is without a doubt one of the most popular retailers in the country, and the pandemic has only increased its foot traffic. Need we mention the insanely long line that trailed outside of that Costco in California back in March when coronavirus made its initial impact on the U.S.?Sales inevitably surged in grocery stores all across the country at the onset of the pandemic as Americans hurriedly stockpiled non-perishables to prepare for the lockdown. And while hoarding behaviors has since stopped, the frequency at which Americans grocery shop has yet to lose any momentum. As is the case for many of those big-name grocers, Costco has benefited from this dramatic shift in consumer behavior and effectively hit higher-than-average sales over the course of the summer months. (Related: These Are the Safest Grocery Store Chains to Shop Right Now).Starting in May, net sales rose 7.5% to $12.6 billion, compared with the same month the year prior. In June sales rose again, but this time by 11% to $16.3 billion. At the end of July, sales had hit $13 billion—a 14.1% increase from the same month in 2019. As of the month that just ended, the company had its largest jump in sales out of the whole summer. In August, Costco's net sales rose by 15% to $13.56 billion. (Related: 35 Cheap Costco Buys That Make the Membership Worth It).Now, when you add up all of the net sales from the 16-week fourth quarter, that total number amounts to a whopping $52.3 billion. For perspective, in the same period last year, the retailer reported net sales of $46.4 billion, meaning its sales increased by 12.7% this year.Costco stands out from its competitors largely because it sells everything in bulk, right? But how about the company's trademark bakery items? From huge sheet cakes to mini cinnamon rolls, it's no wonder the retailer has continued to see so much success during a rather bleak time.For more, check out 5 New Costco Bakery Items You'll Drool Over This Fall.