A baby can be put into a shopping cart seat as soon as the baby can sit up without support or assistance.
Most babies are ready to sit in a shopping cart seat as early as 6 months of age while others may not be ready until 7 or 8 months of age.
So you'll have to be the judge as to when your baby is able to be in the shopping cart seat.
But as long as the baby can sit up without assistance the baby can sit in the shopping cart seat.