You didn’t say what exactly you needed, just the “cart” function or more of a complete online store.
There’s cart2 from @yuzool, it doesn’t have all the features of a complete store, but is a good “cart”.
The complete store solutions are limited to the outside “hosted”, subscription model that Rob mentioned above.
Ecwid also has paid subscription for more than 10 items, a free RapidWeaver plugin and some training videos from @ben. https://rapidweavercommunity.com/addons/plugins/ecwid-online-store