#5: Do you show selectors for quantity feature?
Here is how bad UX looks: If you don't have selectors and don't automatically update the cart, then users will have to do 3 unneccesary actions: Click on quantity input field → Write desired quantity of items → Click on "Update cart" button.
Here is how good UX looks: If you show selectors and automatically update the cart, then users will just need to make one click on selector inputs (plus or minus icons) to change the quantity of an item in the cart.

At the same time, if shoppers want to write certain quantity in input field, they will be able to do that too.

The difference is obvious. Good UX eliminates unnecessary actions and simplify interface, and it's important especially for Mobile users.
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