As Penny says, you have to go through the whole booking to make sure all port fees, taxes, and service fees are in the total. I mentioned earlier you have to be very careful you are comparing apples to apples. You may see an inside cabin on line at $XXX, but is it a bunk bed inside or two lower beds that convert to a King? All cruise lines use categories, ie; "K", "J", BA, etc. or 1A,4A, 4B,8A , etc. If you are absolutely sure you have an apple to apple and it's truly $500 cheaper, than no one can blame you for wanting to book on line. Some on line agencies discount or rebate, and provide little service. If something goes wrong will you have support? Be cautious.