Bonus entry codes differ from regular discount codes, in that they will not be applied at checkout, they will be applied after! Once you place your order and make it to the success page, you'll see your entries, and an option for the bonus entry codes underneath.
Not only that, but if you forget to apply the bonus entry code on the success page, you can also apply the code within your 80eighty account page.
Once an order is placed for the current giveaway(s), you'll see the exact same entry display as you saw on the success page. There, you can click bonus entry codes, and apply your code for 300 bonus entries!