When a customer texts your Grasshopper number, they are simply texting the number, not an extension.
If you only want to receive texts for one of your Grasshopper numbers, you can choose which number(s) that is by going to Settings > Sync & Notification Settings and choosing electing which number you want there.
Voicemails, Faxes and Missed Calls
Within the app, you can choose for which extension(s) you receive notifications for. To change those settings go to Settings > Sync & Notification Settings and check off the boxes for the extension(s) you do want notifications for.