Adding checked bag after online check in?
 

I'm getting ready to check in online for a DEN-ORD flight tomorrow. I've got one free checked bag (through the Explorer card), but may or may not need to check a second bag. I'll figure that out tonight when I pack.

Is there any problem if I choose only one checked baggage during the check in process.. then show up at the airport with a second bag? Can I modify and pay the fee from a check-in kiosk, or will I need to see an agent?

Thanks.

Nope...not a problem. You'll be able to add another bag. You can also not check a bag you said you would (by just bypassing check in).

Don't know the answer to the kiosk vs. agent question, but I suspect the kiosk will give you an option to change your bags (like you can in the app after initial check in). On that note, you can always go to the check in tool to amend the number of bags you are checking.

Note that I don't know how well it does with this and a payment required - due to elite status, I haven't paid checked bag fees to UA for a loooong time..possibly ever.

No problem - even if you're already checked-in, the kiosk will allow to select/update how many bags you're checking.