It's an interesting question, but if I were to look at it from Viking's perspective, I might get to Day 14 and decide I could save money by buying the SSBP. I doubt they would let me do that. Although, looking at the "rules", I can't see anything that says I have to buy it before a certain point. I will be interested to see if anyone here has tried to do this.
Personally, if I thought it was going to be a close call, I would just buy it and forget about it. One less stressor on a holiday.