For Burlington Trailways buses, is advance booking helpful and needed to be sure of a seat?

Later this spring, I will probably travel by bus from Mount Pleasant to Iowa City (I'll be arriving by train from Denver). According to Burlington Trailways, there is a ticket counter at the Mount Pleasant bus stop. I could buy a ticket in advance for $18.50. Is it worth booking this ticket in advance? In general, I can think of two reasons to book in advance:

• Buses may get sold out. Is this a significant risk? The complete route is Chicago–Burlington–Mount Pleasant–Iowa City–Des Moines–Omaha. How full is this bus, typically?
• Tickets may get much more expensive. The same bus is also listed on Greyhound, but out of Advanced Purchase, Web Only, Standard, and Refundable, only Refundable is listed. The total distance is 50 mi / 80 km and the listed price is $18.50. Will I pay much more if I book at the counter just before departure?
• Thirdly, do I even gain anything? Does booking in advance guarantee a seat?

One argument against booking in advance is that I hope to meet someone on the train who's also travelling to Iowa City, and that I can possibly share a ride. Then I won't need the bus and will be in Iowa City much more quickly (it's five hours between the train arrival and the bus departure — that's good if the train is late, but a waste of time if it's not). If I book a ticket and not use it, I still need to go through the effort of cancelling it. Even if I can do this after the fact (that would surprise me), it's likely more effort than buying a ticket just before the bus departs.