My suspicion is that Costco chiseled the price down to where Solomon’s makes very little per lb. Costco orders what they want, but Solomon’s only fills the order when they can fill all of their other customers’ orders as well.
In times of high demand, e.g. before Yom Tov, Solomon’s can’t fill all orders and will give priority to their customers who pay their regular price.
Again, just a theory.