Amazon gives non-Prime members free shipping at $35 or more of eligible items. Instead of simply letting users get the product with free shipping, they’ve added a discount that prices it exactly one cent below the $35 limit, while only subsidizing the price with $3.38, which is about half of what they’ll then charge you for shipping.
Just a heads up on the Costco online thing, their selection isn’t that expansive, so it can’t replace Amazon for you. You’re at the whims of what item and brand they decide to carry and when.