I have built a few Post paid POP's using Mikrotik User Manager and some monthly reset scripts that i found in the wiki.
http://wiki.mikrotik.com/wiki/AutomatedBilling
I have been using it for PPPoE & Hotspot purposes and i have it uploading daily usage to a separate billing engine
I have customers i shape automagically after using their monthly quota using Usermanager and then these scripts reset the quotas and there connections at 10 minutes to midnight on the last day of every month.
You will need to test, tweak, and modify to your requirements, also the syntax is a little dated. I have been using it for a couple of years now on various models and ros versions including RB2011, CCR1009, ccr1036 and ROS 5.xx and 6.xx
Good Luck