This one folds into a teensy pouch when not in use, is tear- and water-resistant, and even has a separate side pocket for shoes. Aaaaand it has a strap on the side that slides easily over your roller luggage handle.
Just keep it in your regular suitcase between trips, and you'll never forget it. Fold-up duffels like these are my GO-TO when going on a vacation I know will be shopping heavy. And then you can bring your new finds on the plane to ensure they don't get lost (it's 20 inches, making it perfect for the overhead compartment).
Get it from Amazon for $17.99 (available in 25 colors and patterns).
Promising review: "We bought this to take with us when we visited our daughter who is studying abroad for four months. We figured she would not have enough room in her luggage when it was time to come home. Folded up, this bag was so compact and lightweight it took up almost no room in our own luggage for the trip over. Then, sure enough, she had a ton of clothes and shoes to send home with us and packed this bag full. We checked the bag and it made it through two international flights without a scratch. The BEST feature is the side strap that allows you to slide it over the handle of a rolling suitcase so you don't have to carry it. This is probably the most practical and the best-value item I have purchased in years." —Tosa Mom