It must have been quite a shock to receive such a large bill for your first month's service with (name). When you signed up for our service, you agreed to pay $(amount) for every minute you went over on your cell phone usage. I understand monitoring cell phone usage...
Dear (recipient's name),
It must have been quite a shock to receive such a large bill for your first month's service with (name). When you signed up for our service, you agreed to pay $(amount) for every minute you went over on your cell phone usage. I understand monitoring cell phone usage can be confusing, especially when you are a new user. I'm willing to take the extra charges off this month's bill, but expect you to monitor usage on your own in the future to avoid any further charges.
Sincerely,
(your name)