A baby can be put in the shopping cart seat as soon as the baby is able to sit up without any support or assistance on their own.
Usually this happens by 6 months of age which is when you can sometimes put the baby in the shopping cart seat.
Some babies may not be ready to sit in the shopping cart seat until 7 or 8 months of age though but as long as they can sit up unassisted they can then safely sit in the shopping cart seat.
Just be sure to keep an eye on the baby and buckle them into the shopping cart seat for safety.