SoCal

A travel agent said you'd go through Immigration and Customs at DEN???? I wonder if they deal much with people buying international flight tickets. Tell them to phone United Airlines to confirm. If they won't, do it yourself. I've never heard of such a thing. Presumably the agent would not have been able to book the flights if 1.5 were not a legal connecton time for international-to-domestic flights at LAX. It sounds like a tight connection, but if it's a legal connection time I doubt UA would change it without a fee.

Unless you have a truly uniique situation, it'll be at LAX were you'll go through Immigration (that's the wild card since you don't know how many other flights arrive at the same time and how many people will be in your line-- citizens/permanent residents or others), then to the baggage carousel to get your bags, then Customs (usually just a matter of handing over the form you will have been given on the plane and filled out and had initialed by the immigration officer, but you could get pulled aside for secondary inspection, whether due to questions about your form or bags or just at random), then you hand over your bags to a transit desk, then proceed to the gate for your next flight, after passing through security. Good point about remembering to put into your checked bags anything you bought in duty free shops that isn't permitted in carry-on bags for U.S. flights (my wife forgot about some hand creme she bought in duty freeon a flight from Brazil and forgot about it until after we'd handed over our checked bags at the transit desk and were passing through security at ATL-- she lost the creme).

If somehow you miss your connecting flight, and that's a possibility with such a short connectiontime, go to a UA customer service desk in the terminal. They will put you on the next flight that has available seats in your fare class. That may not be the very next flight. Since DEN is a UA hub there will be lots of flights, so I doubt you'd have much of a wait. If you have to overnight, UA may pay for a hotel, but only if the delay was their fault (not due to weather, Immigration delays etc.).