You don't see how adding functionality that requires writing to the database rather than just reading from a cache could "drastically increase database load"?
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.