As long as the merchant has the setting turned on, it will appear for all buyers at the checkout. Hope that helps!
Ah, I see. At the current time this is only setup for buyers in Canada, the USA, Australia, and the UK, so if your customer is not in one of those countries they won't see the option. If they are in those areas and still aren't seeing the option, definitely contact support and they can take a look for you. Cheers!