Deja, I echo Kuki and Donna's statements. Worst case you just cancel and re-book, but be sure and check if they include port charges. That's how they do a lot of come-on fares.
On your other question, I just went Princess in February and I don't recall any $100 bonus from them doing things on-line.
Don't let it spoil your trip ... one way or another you got a great rate, but I agree with you, save where you can.