Just another point - if you go to Gouvia to book, you will need to pay cash - they cannot take credit cards at Gouvia.
Also, as you have found out, they are not the quickest at answering emails. I did eventually get a reply to say they had passed my email on to the Corfu office, but they never answered it. Eventually I went into the office in Paxos (where we were staying for the first week of our holiday) and "persuaded" them to take my payment by credit card over the phone, as I did not have enough cash to pay for 3 tickets in cash. The flightseeing tour is currently 89 euros (or 99 if you book transport from your accomodation). To be honest, you probably get a better deal flying in and out of Paxos on the scheduled flights.
You will enjoy it tho.