Can I book checked baggage for United and Air Canada?

When booking an international flight, you can add checked baggage for approximately €60 per person per flight. However, I'd like to decide later whether to fly with or without checked baggage. In particular, I might only want to take one suitcase back on the return flight (for a shopping trip).
Unfortunately, I can't find anywhere on the websites whether I can add checked baggage at any time for the same price.
Does anyone have experience with this?