If you've already paid extra for delivery and set-up, I'd say $10 or $15...
When we moved we bought a lot of new furniture, all from the same place. We paid only $50 for delivery & set-up of EVERYTHING (bedroom set (bed, bedside tables, dresser), tv armoire, coffee table, end tables, dining room table & chairs, bar stools) and gave them $30 tip. They were here for about an hour (plus we live outside of the city). We thought that was fair and they seemed VERY appreciative of it.
Because you've already paid extra, I think $10 or $15 should cover it. BUT that's just my opinion!