Just tried to use that and they are charging me NZ$22 with the coupon applied, which is north of US$15.
The cart doesn’t break down the discount at all, so it’s hard to see what’s what.
Edit for those paying in other currencies. From Humble Support:
The coupon in question provides a discount to the USD price of the Civilization VI - New Frontier Pass, bringing the price to $10. This comes to a total discount of $29.99 USD. $29.99 is then converted into the currency to which the account is tied. Instead of seeing the equivalent of $10 USD, you receive the same discount of $29.99 USD towards the total price of the item.
I want to apologize for any frustration this has caused. We could have been clearer in our messaging, and we are taking steps to avoid this sort of confusion in the future. We do hope you enjoy the coupon, and please reach out if you have any further questions or concerns.