With orders of $25 or more, Amazon offers the AmazonBasics Strap-On Wedge Bike Saddle Bag in Small for $2.12 with free shipping. (It's available in larger sizes for at least a buck more.) That's $8 off and the lowest price we could find. It features a reflective safety strip and weather-resistant material.
Note: This item is offered as part of Amazon's "add-on program" and can only be ordered with a purchase of $25 or more.