At the Mittaphap Lao-Thai Bridge, on the Lao side today, there's a large duty free shop. It's pretty but kind of pricey. Instead checkout the surrounding smaller vendors where they have much better deals.
Finding lots of cheap cigarrettes, cheap alcohol, not to mention namebrand imitation clothes, cologne/perfume, and bags. Gets quite busy in the afternoons with Thais and other foreigners here for deals.