Consider the UML use case diagram below. The diagram shows that

top level use cases are View Items, Make Purchase, Online Shopping and Client Register.

Make Purchase use case could be used by customer as top level use case if customer only wants to find and see some products. This use case could also be used as a part of View Items use case.

Checkout use case is included use case, so it is not available by itself, as checkout is part of making purchase.

the Online Shopping use case allows customer to register on the web site, for example to get some coupons or be invited to private sales.

PayPal actor uses some web site to make purchases online.