Yes, you can check in with bags on-line. The process will ask you how many bags your are checking via a pull-down menu. You then print your boarding passes. I suspect you will be able to pay the luggage fees at that time via credit card.

When you arrive at LAX, Terminal 7, you can either show the Skycap your boarding pass and have them check in your luggage (with the appropriate fees) or there should be a seperate set of kiosks in the lobby area that are marked for "ual.com passengers with luggage" or something similar.