You can buy bus tickets for Boston service ahead of time but it's not required and seats are not guaranteed.
Reservations are neither needed nor accepted for C&J’s bus service between Dover, Portsmouth, and Newburyport and Boston’s South Station and Logan Airport. Passengers can buy tickets in advance, but advance purchase does not reserve a seat for a particular bus. Travel aboard C&J’s non-reservation service is on a first-come, first-served basis. Tickets may be used at any time for the South Station and Logan Airport scheduled service.
All tickets are open-ended with the exception of multi-ride passes and same-day round-trip tickets. One-way tickets are valid within six months of the date of purchase. Refunds are available prior to the date of expiration and are subject to a $5 per-passenger refund fee. Contact the C&J Information Center at 800.258.7111 or send an email to firstname.lastname@example.org to initiate a refund.