JetBlue Plus Card Bag Fee Waiver

Does anyone know how this works? I've got 4 trips with multiple travelers coming up and I expect to check 3+ bags each one. Bag fees are $70 and $80 roundtrip and there's about 12 different times I'd pay the fee. If I got the card, would it waive the fee? I've already booked the flights (on points) and didn't have a JetBlue Plus card. All the documentation I've read says I have to pay for the flight with the card. Does it actually work that way or do they waive it if it's somehow in my profile like American Airlines?