On a coupon or punch card, under How customers get it, set Ask for contact details to Optional or Required, and choose email or mobile. The public link then asks before handing out the card. Customers who got their card at the till can add an email on their card's page.
With Email reminders on, customers who gave an email get one email when a coupon that opens later starts working, and one 3 days before a card runs out (a punch card only if it has stamps on it). Reminders go out in the daytime, once each.
Each coupon and punch card page shows how it is doing. For coupons: how many were used, how soon after they were given, how many ran out unused and where they were used. For punch cards: how many holders came back, visits per card, the usual gap between visits and how many reached a reward. Stamps less than 15 minutes apart count as one visit.
Download contacts (CSV) lists everyone who gave details. Only send marketing to those marked yes.