Text messaging does not use data. Sending a text message (through your phone's text messaging app) to an e-mail address does not use data.
Checking your e-mail through your phone's web browser or an e-mail app DOES use data. Android phones, by default, are constantly connected to the network, looking for updates for your applications, and also downloading information for those apps (news stories, weather reports, etc.)
If you want to stay under your 150MB limit, you'll need to find instructions on how to disable 3G data on your phone and only enable it when you need it. Otherwise, confine your data usage to WiFi networks (which doesn't count against your data package). Androids don't compress data like Blackberries, and they can very easily go over the limit of the $15 plan if you aren't diligent in monitoring your usage and keeping 3G data turned off.
If you don't want to monitor your usage, you can upgrade to the $29.99/Unlimited data package.