There are several tour companies that run from Seattle to Mount St. Helens. All of them are long but beautiful and informative. I recommend you try Seattle Tours for a smaller more intimate tour of 1 to 6 people. Call 1-800-305-9617 to book.
Yes, call 800-305-9617 for reservations for a bus tour. Bus leaves daily at 7:00 am. The 800 number will help you find the closest pickup location for you. The cost is $ 116. 95 per person based on a bus with 6 passengers. The tour takes about twelve hours.