If the feather is why you are going to the vet, I don't think you will need to by tomorrow. Just get a pair of pliers and yank it out. It's surprisingly easy. Or, you can use pliers to just pinch it off.
Idk about pulling it out, I dont want to end up hurting her more if I do something wrong, I'm new at ducks. I've also been hearing them cough so I wanted her to check that out too. Thanks for helping everyone!!