With purchases of $25 or more, Amazon offers its Prime members its AmazonBasics 13.3" Laptop Sleeve in Blue or Purple for $2.52 with free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's $8 off list and the lowest total price we could find. It features a top-loading zipper.

Note that this item is sold as part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.