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.