As we entered the town of Tupiza, where we had to stop at this checkpoint while our driver got hassled by the police, we were afraid that we had missed the bus to the border town of Villazón. As we pulled into the bus station, a bus for the border was about to depart. We ran and managed to get on it just as it was leaving. We were jostled around in the back of the bus for the next 2 hours, but we DID manage to get to the border and found a place to stay on the Argentinian side in the town of La Quiaca.