We have a similar setup. 15 day trial with cc as per of registration. We sent a few emails in thos 15 days. 1st is the welcome. If they haven't logged in more than once we send another email on day 7 to be sure see the value and if they need help and about their trial. On day 14 we send a reminder about their trial. Day 15 we charge. We will gladly refund the charge up before their 2nd due date.