The email must be related to the contest and specifically to their chosen charity in some way. We also are not allowed to give out cell phone numbers. That's the agreement that's made when people agree to the terms. So, they'd have to do something like: "We'll donate an extra $5 to X charity for each coupon redeemed in the month of August." or specifically mention that each coupon redeemed gives $15 back to the charity. Other than that, there are no other rules I am aware of. As for other ideas, here is this list of 101 things from Matt Michel