Just to clarify for jweiss-when booking air directly thru the cruiseline, AAA or any other agency for that matter, has no control over the flights. You can pay an air deviation fee which is $50.00 per person AND you also have to pay the difference between the flights that are chosen and the flights that you would be booked on if applicable. This could or could not cost you alot of extra money. With this option, you can choose which airline or flights you would like.
Again, this has nothing to do with AAA.
And to answer the question regarding combining shipboard credit, for the most part you can. There are circumstances, I'm sure, in which credits cannot be combined but they are few and far between.
I myself, have combined both a rebooking discount w/ the AAA discount without a problem.
Unless a TA is offering a cruise @ a group rate or if the TA is taking off a large chunk of the commission, the rates should be around the same.