García, who homered and drove in five runs in a 9-5 win Saturday, and Shaw hit early drives that helped the Brewers build a 7-0 lead in the second inning.
Brett Anderson (1-1) tossed five effective innings for Milwaukee, which has won four of five.
Austin Dean had a two-run double for the Cardinals, who have lost two in a row after a four-game winning streak.
The Brewers scored four times in the first off Daniel Ponce de Leon (1-1) in batting around for the first time this season. Lorenzo Cain began the game with a first-pitch double, Daniel Vogelbach followed with an RBI single and García homered.
Shaw hit a three-run homer in the second off Ponce de Leon in the second. Shaw left the game as a precautionary measure after fouling a ball off his shin in a fourth-inning plate appearance.
Milwaukee outfielder Christian Yelich left the game in the second inning with back soreness.
Piña added a two-run homer in the ninth.
Anderson allowed one run on four hits. He struck out one and walked two.
St. Louis newcomer Nolan Arenado extended his hitting streak to nine games with a first-inning single.
Daniel Ponce de Leon on today’s start: “I felt like my arm was like 100 pounds — not 100 pounds, but real heavy. I just could not get it out in front. Just using my whole body to get a strike, and I just couldn’t get it.”#STLFLY pic.twitter.com/0aqFoiDyY7
— Bally Sports Midwest (@BallySportsMW) April 11, 2021
St. Louis C Yadier Molina batted in the cleanup spot on Sunday for the first time since June 2, 2017, when he went 0 for 3 in a 3-2 loss to the Chicago Cubs. Molina went 1 for 3 on Sunday and is hitting .367.
Brewers: Milwaukee INF Jace Peterson made his first start of the season, replacing Kolten Wong, who is on the 10-day injured list with an oblique issue. Peterson underwent cryotherapy on a sore thumb after spring training was completed. “It seems to be working,” he said.
Cardinals: OF Tyler O’Neill was placed on the 10-day injured list with a right groin strain suffered during his first plate appearance on Saturday.
Brewers: RHP Freddy Peralta (1-0, 0.00) will take on Chicago Cubs RHP Adbert Alzolay (0-,1, 7.20) in the first of a three-game series on Monday in Milwaukee. Peralta is 3-1 with a 3.78 ERA in seven lifetime appearances against Chicago.
Cardinals: RHP John Gant (0-0, 0.00) will face Washington RHP Erick Fedde (0-1, 27.00) in the opening game of a three-game series on Monday in St. Louis. Gant has not allowed a home run in his last 39 1/3 innings, the longest current streak in the NL.
Source: Fox Sports