BOGO (free)

BOGO – “Buy One Get One” type of discount.

Look at the examples:

1+1 Product (Buy 1 product and get the second one for free)

Buy any product with certain attribute and get a certain gift

Buy 2 products from category and choose your gift from another category

Buy 2 certain products and be sure to choose a gift

Draw your attention to the drop-down’s value “Require to choose from product/categories”. Despite of “Allow to choose from products/categories”, user can’t place order without gift.