Buying Plus Fare onboard

I want to upgrade our fair to the Plus package but can't because we've already paid our fair in full. Customer services says we can just do it when we board, but I'm very concerned the price will be double when we do that.

Right now, the app says the fare is "on sale" for $60/day (normal price $120). My impression from the Internet is that it's always $60, but I really don't want to pitch up and be charged $120 on board instead.

Is this likely?