With such a big margin to overcome, Bolivar had attacked from the start, hoping to take advantage of the tough conditions their home ground - 3,600 metres above sea level - creates for visiting teams.
However, they created few chances of note. Gerardo Yecerotte's right-footed volley settled the score on the night in their favour, but other than a Juan Callejon free-kick which bounced narrowly wide and a Jose Chavez chance in the second, they showed few signs of clawing back the deficit.
"This is the prize for a big effort," San Lorenzo playmaker Leandro Romagnoli told Fox Sports. "We have one little step left to make our dream come true.
"We all want the cup. This is the first time the club will play in the Libertadores final and we have what it takes to win."
Bauza will be trying to win the Libertadores cup for a second time, having tasted victory with Ecuadorian side Liga Deportiva Universitaria in 2008.
The first leg of this year's final will be played next Wednesday in Asuncion, with the return leg in Buenos Aires a week later.