It's been possible to buy this service without United's help for a few months now. If you sign up on their website, you can order services from airports that United is not yet supporting directly. I haven't tried it yet (as I rarely check luggage), but I've been tempted given the pretty reasonable price.

It's also worth noting that the order deadline is only an hour before the departure of the last flight on your itinerary ... when I contemplated doing it, I almost placed the order during a layover in Houston because the thought of lugging bags from SFO through BART at rush hour didn't sound attractive.