If you don't get it settled up, and you go to work for another ALPA carrier in the future, you won't be able to join the union until you settle the balance. They can get a payment plan setup for you so that you can pay it over a longer period of time to help. But without pay stubs, I'm not sure what to tell you about how to dispute it if you think it's incorrect. Your best bet is to just call Membership Services and see what they say.