I highly recommend the FilliMilli Mini Brush Set! I didn't realize how important brushes were, but I bought this set because it was on sale, and it turned out to be so good that I'm using it a lot now! It includes a narrow brush, a contouring brush, and an eye brush, so I find it very useful. Since it's small, it's convenient to carry around when traveling. I totally recommend it! It even includes a powder brush, so I don't bother packing a separate one; I just take this whole set with me. It's a great option to carry around when buying individual items feels too expensive.
