It's probably the $15/month plan for 150 mb of data.
If you download the My Verizon app, it'll give you your account status, including number of talk minutes used that month, number of text messages sent, and amount of data used (in kb).
Keep an eye on apps that automatically sync. Also, download and update apps over wifi so you don't use up your data doing so.
Honestly, for a phone like the Dinc, the unlimited plan is very handy. I checked my data usage at the end of last month, and it was well over a gigabyte. It might be tricky keeping it under 150 mb, and if you go over you end up paying ridiculous overage charges (something like 25 cents per megabyte, probably rounded up since that's what they do for minutes).