It'd appear that your cruise is made up of a TATL portion followed by a European cruise. I'd wager the TATL is not as sold as the European portion is. You might want to look into pricing the two cruises separately vs. as single cruise (may be differences in fares, taxes, etc.), though you might have to change cabins. Might be able to get a better cabin for the TATL that way.
As for where to book, I attended an Oceania presentation by supposedly one of
their largest agents in Canada. Some cruisecos may give cruises purchased in Canada a break on the exchange rate (I've see up to 10%, e.g., pricing the cruise at parity when the CAD was trading at USD 0.9,
etc.).