German rail fares

We need some advice from someone with experience of German railways
and their fare structure. Hopefully someone on the newsgroup can help.

We need to travel from Frankfurt Airport to Villingen (Schwarz) with a
24hr stop in Heidelberg. We've checked out the trains on the excellent
German Rail website and found a suitable service for our purposes, and
also the price of the ticket. What we don't know is: Would the price
be the same if we just turn up at the airport station and buy the
tickets there, or am I getting a special deal on-line? Our date of
travel is three weeks away. The advantage to arriving and buying the
tickets is if our plane is late in which case we could miss our booked
train. However, if it's cheaper to buy the tickets on-line I'll do
that and risk the plane being on time. (They always are unless you
really need them to be...)

By the way, the service I'm looking at is the regional service via
Mannheim, and the on-line price one way for two adults is E55.20

Anyone have experience of this situation?

Help apprectaied,
Paul
New Zealand.