Looks like you can pretty much get 20% off if you buy 2 Lego sets the same value.
Also some good other toys included.
Update: According to tests and purchases (mine included) it currently becomes a flat 20% discount off all items.
- Save 40% off the second item when you purchase 2 items across Toys, Lego, Costumes, Kids Art & Craft, Books and Paperchase.
*Purchased in one transaction. Not in conjunction with any other offer. Excludes: Clearance, Board & Card Games and selected LEGO ranges (Art, Creator Expert, Ideas, Super Mario, 71043 Harry Potter Hogwarts Castle, 76139 Batmobile and 42115 Lamborghini Sian FKP 37).
You just need to buy two of ANY item part of the promotion to be eligible for the deal. I bought the a- wing and combined it with a $9.95 matchbox truck also part of it.