postgresql – Should we favor multiple INSERT or multiple UPDATES

I always have the impression that INSERT would be beneficial for the performance (forget the storage for now) instead of UPDATES, in all the DB that I know. Especially in Postgres, UPDATES involve transactions and may result in blockages

Imagine that we are building a shopping cart. For each element I have a table with these fields. [row_id, product_id, quantity].

Should I model data to UPDATE (I update the quantity column several times when the user adds / deletes from the cart)

[row_id = 1, product_id = A, quantity = 3]

or to model multiple INSERTS (insert a line for each time the user clicks and deletes everything when they are removed from the cart) as a flow of events

[row_id = 1, product_id = A, quantity = 1]    // user clicks add to cart
[row_id = 2, product_id = A, quantity = 1] // user clicks add to cart
[row_id = 3, product_id = A, quantity = 1] // user clicks add to cart
[row_id = 4, product_id = A, quantity = -1] // clicks of the user remove from the cart