Adding to this (maybe this thread could become a sticky for bike-on-bus services?), Bolt Bus also allows bikes in the lower storage of the bus. I have used it twice with no issues whatsoever going between DC and NYC.
From their site: https://www.boltbus.com/faq.aspx
Can I take a bicycle or golf bag?
We do allow oversize items as long as space is available in the baggage bin. The oversize item will count as your one bag allowed under the coach. You’ll be responsible for placing the item in the baggage bin. We will not charge you an additional fee for oversized item provided it is within your limit of one bag under the coach.
I would add a word of caution, though: there is nothing stopping your bike from bouncing around in the baggage bin, so make sure all breakable parts are protected.