Each buyer starts with an empty cart which then accumulates the items they've added to it -- just like a physical shopping cart in a real-world store. Each buyer's cart remembers its contents for up to 24 hours after their last cart view. We set a cookie in each buyer's browser to keep track of which cart instance is theirs, so using a different computer or even a different browser app on the same computer would each be tracked as separate carts.
Thus, if you'd added an item to your cart for testing yesterday, that item would still be in your cart when you resumed testing today with the same browser app on the same computer, but that earlier item would not be present in the cart when you use a different browser or computer.
If you prefer to send buyers directly to instant checkout for just one item at a time, you can use our Buy Now buttons instead of Add to Cart buttons. Note that Buy Now buttons cannot use some features that depend on the cart, as we describe in more detail here: