Buses (besides greyhound) to get from Boston to Seattle?

So my friend is supposed to come to Seattle (or Everett, etc.) but she's never flown. So she's looking for alternative routes. Amtrak is super expensive for such a far trip. Are there any perhaps lesser-known companies that can get her from Massachusetts to Washington? Transfers are probably inevitable, but we're looking for cheap. Any suggestions? And we are well aware of Greyhound's existence, so please leave that one out. Thanks! Oh, and links are appreciated if possible. Thank you!