Your unused Booster will carry over to next month if you renew your base plan. So, if there is enough amount in your account : $15 + tax in your case, base plan will renew every 30 days. And if you have some unused minutes or data left, then it will carry over.
They have double value promo on Booster right now. Grab it if you know you will use it. Prepaid doesn't get promo that much. You never know when they will offer good promo for prepaid next time ;)