I did not used Google Sheet as Database and I dont see it as a good option for connecting to it via a programming language when taking into consideration any RDBMS or NoSQL if that is needed.
I recommend either PostgreSQL or MongoDB or Cassandra.
An Account has multiple shopping Carts.
Some Shopping Carts will transform in Orders.
Both Carts and Shopping Carts will have many items/products.
...
So you will need to have one to many or many to many relations. It is easier to do it with relational database in my own opinion.
I prefer to use PostgreSQL. It is easy to install and works great for majority of cases. But you can also use MySQL. It will get the job done.
Both are well supported in most programming languages.
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