You can select Free Gift option on first step of offer setup, Free means 100% discount on offer. Customer will get a free offer after payment page on checkout.

There’s 2 options to offer a free gift,

• Choose discount type percentage and set 100% in the discount value field.
• Choose a discount type flat and set the same product price value as the discount value.
  In both the cases product price will be 0 and customers will get that offer free!

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