Baytril is what I would use, but as you said, it has to be ordered. How nice of you to offer to share yours. I hope you two are close enough to make it work. :hugs
Welcome to BYC. Corid can be given to ducklings if they have coccidiosis, but it will not treat a respiratory infection.
Questions for you:
Where do you live?
Do you have a food scale that you can weigh the duckling on? If not, can you get one?