My app has a 7-day trial, and requires a credit card up front. I try not to beat people over the head with a note about an upcoming charge - what I do is, 3 day

My app has a 7-day trial, and requires a credit card up front. I try not to beat people over the head with a note about an upcoming charge - what I do is, 3 days before the trial ends, I send an email asking how the trial is going, and if they have any questions I can answer. It serves as a subtle reminder that the trial is about to end, while offering to help at the same time.