Au Mobile Plan Cancellation

I came to Japan nearly 2 years ago and got a mobile plan from Au. The 2-year mobile plan will end this July. However, I have to go back to my country for good by the end of March. By this I have to request Au service disconnection by the end of March.

My questions are,
1) When should I advise Au my request for disconnection? Do they require 1-month or 1-week notice prior of leaving Japan?
2) Au's billing cut-off is on the 25th day. Can I request them to extend my usage and billing until March 31?

I know it is best to inquire this directly to Au.But I don't have a japanese friend with me now that could help me for translations. I'm quite worry of my basic nihongo if we come up to talking some technical terms and conditions.

I am first hoping a help from this site.
Thanks a lot!:)

You can break the contract immediately as you visit au shops. It will cost you a penalty of 9,500 Yen. This is because you will break the two years contract before it matures.

You can't use the phone until the last moment in JP. Your bill of payment must be completed before you depart.