I was not able to pay for my checked bag ahead of time on my international flight. They need to weigh it and check passport.
I could check in because my whole flight (both legs) was on the same carrier (American), however my daughter’s international flight was on British Airways and then American, and she could only check in for the British Airways leg. When she got to the airport, and everything was verified/weighed, they were able to print her boarding passes for both legs of her flight.