You can setup an automatic warning when Verizon thinks you might go over your limit.
If so, you can increase your limit for the current month to avoid the overage charge.
When you do so, be sure and reduce your limit for the next month. If you don't you can't cut it back until the following month - which would charge you more for that next month even if you don't hit over your planned limit.
But if you keep cutting it down next month, and only increasing it during a month when you are in danger of overshooting your limit, you will keep your data costs down as much as you can.