Even if it's writing to the DB, I doubt basic queries like adding an item to a cart is going to need DB sharding. I think one piece that someone else mentioned

Even if it's writing to the DB, I doubt basic queries like adding an item to a cart is going to need DB sharding. I think one piece that someone else mentioned could require DB sharding and that is all the live data needed for tracking deliveries. The actual website/app should not need more than one beefy Postgres instance.