wongtop: If you've got lots of ups and downs over the year, you could look at some of 2degrees plans which would allow you to carryover minutes when you don't use them.
^ This looks like a good answer, Our Business plans have the carry over mins and data that would add up over your slower periods and be available for your busy periods. The oldest mins/mb are used first and wont expire until they get to 12 months, I often have customers coming in with 2000+ accumulated mins on their phone. Plus we offer exactly what you have asked for with our spend control - It notifies you when you hit a $ amount that you have specified and stops your usage until you opt to exceed it . We also send a text notification when mins or data are running low. And we do sell extra minute packs to both prepay and pay monthly customers so you can avoid the casual rates.