what happens after Sept. 30 2013, do I have to pay?
They said I was given 5GB extra storage until Sept. 30, 2013. What happens after that?
At that point your extra storage will cease and you will revert to the free 5GB, unless you decide to pay to continue the upgrade. You can purchase additional storage if you wish to, but note that this will over-ride the free storage. The additional iCloud storage levels available are: additional 10GB, $20 p.a., 20 GB, $40 p.a., 50GB $100 p.a.
More details in this Apple Help page: http://support.apple.com/kb/HT5527