Be very careful about using online travel agencies, especially in these tough economic times. Their financial stability is probably a mystery....
Last spring I searched 14 different online agencies for the lowest fares on two Holland America ("HAL") cruises, New Zealand-Australia in 01/09 and Scandinavia/Baltics in 07/09. I booked both cruises with Cruise Value Center ("CVC"), one of the largest online agencies, which had the lowest fares for both, meaning they were sharing more of their sales commission with customers than other agencies do.
In about August, CVC started keeping credit card payments from customers instead of promptly passing the payments on to cruise lines or (as most agencies do) charging the card in such a manner that the credit card company immediately pays the cruise line. The extreme discounting had apparently caught up to CVC and ultimately, they went out of business last month, leaving well over 1,000 customers without fully paid cruises, even though they had sent customers (as they did for my 01/09 cruise) a "paid in full" receipt.
I was lucky. I saw a thread on Cruise Critic about CVC's demise, checked with HAL ("Sorry. We haven't received final payment"), paid HAL $$$ and filed a complaint with my credit card company, fairly quickly getting a permanent credit for the amount in dispute, but only because I caught it within the 60-day limit after the company mailed the credit card statement with the charge on it.
So, the moral of the story is that, if you decide to use a travel agency, please be sure to check with the cruise line to ensure that they receive full payment. For HAL and many other cruise lines, there is an easy way to confirm the final payment. The online check-in option offered by many cruise lines (including HAL) usually isn't available until final payment has been made to the cruise line. That might uncover a missing final payment but wouldn't uncover a missing deposit.
In my first 18 months of retirement, I'll spend 107 days on eight cruises. Two of those cruises almost turned out to be much more expensive than I planned! Both are still somewhat more expensive than I planned, because HAL didn’t know how much the steeply discounted CVC prices were and, not surprisingly, wouldn’t honor those prices.
P.S. HAL is my favorite cruise line, but I'll also cruise on Princess, RCI and NCL in this 18-month period. Four of the eight cruises are on HAL.