Also, what does grass-fed really mean?Credit: Instacart / AllrecipesWarehouse giant Costco might not be the first place that comes to mind when you hear the words "luxury" or "indulgence," but its butter—specifically the Kirkland Signature Grass-Fed label—hits those marks.Creamy, versatile, and affordable, this dairy product ranks high on the list of Costco items with a cult following. Some say the European-style butter is similar to Kerrygold, …