On most flights, UA won't complain if you'd like to pay them a lot of extra money for excess baggage. On some flights, particularly around holidays, they may refuse. That's more common on Central American routes, though, which are generally flown with smaller planes. I've never heard of a luggage embargo on a Europe flight.

If any piece is oversized or overweight, the $200 stacks with the other charges. So, if you bring three pieces, and one of them is overweight (23-32 kg), you'll get 1 free from the fare, then $100 for the second bag, and $400 for the third bag.