You don't pay for an email address, because thanks to email interoperability (not openness, which is a separate albeit related thing) a free email address is a decent substitute for a non-free email address. Viber is not the same kind of substitute for WhatsApp when all your friends/family are on WhatsApp. (I live in a market where everybody is on Viber btw, so the WhatsApp card is worthless even if they paid me $0.01 for each WhatsApp message sent)