I feel the best way to inform users is to send them an automated mail - 24 or 48 hours - before actual charge and tell them explicitly how many hours before upc

I feel the best way to inform users is to send them an automated mail - 24 or 48 hours - before actual charge and tell them explicitly how many hours before upcoming charge, which makes it easier for a user to figure out actual time of the charge irrespective of their timezone.