Creative ways to reset a user each month?

So perhaps someone might have a creative solution to this:

We allow our users 3Gig of free data per month, as per expected sophos handles this nicely, and will reset the users counters at the end of the month, giving them another 3 gig's for the new month. 

But....We also allow people to purchase more if desired.  So each user has 3gig default, but can purchase additional gigs if wanted,  so for example, if a user wants another gig, we change their "network traffic" to 4gig, giving them the extra gig they want.  On the first of the month,  we change everyone manually back to the 3gig (otherwise users network traffic we have upped, will allow them the network traffic per month that we changed it to,  as in the example, the user we upped, to 4gig per month, if we don't go in and switch them back)

I'm wondering if there might be a better way to go about doing this, perhaps some way of automatically changing them back to a default setting, or perhaps an even more interesting way of accomplishing what we are doing.  

thanks for any input anyone may have.