Parts of my Medallion app are not working so I logged in to the desktop version of the Princess website a couple of days ago to see if I could finish my check-in there. When I hit the "onboard services" tab within the "Manage Your Cruise" section, it brought up a green button that said "add Princess Plus or Princess Premier to your booking". Hitting the green button brought up a page with the option to "upgrade" to Princess Plus for $50 p/p per day or Premier for $75 p/p per day. Even with our Elite discount on wifi this seemed like a no-brainer for tips and drinks so I read the FAQ and terms but couldn't see anything that said whether it was refundable. The purchase process is similar to, say, buying a shore excursion or specialty dining add-on so I that's why I was confused whether this was a refare or just a refundable add-on.
Now that I've gone through the process I can confirm that it is being treated as a refare. After purchasing, I received a new confirm with the tips/drinks/wifi noted and the additional $700 we paid for the Princess Plus "upgrade" (2 ppl x 7 days x $50/day) was added to our base fare. The $700 was also immediately charged to our credit card. I don't know if it's relevant but our cruise leaves Oct 8 so we are about 5 weeks out.
Thanks for everyone who took the time to weigh in here.