If you are going to use the sandbox, use the member ID that was given to you when you signed up for the Sandbox.
If you are not going to use the sandbox, use the Live member ID.
If you do not wish to use the Paypal Express Checkout, you can delete the Express checkout button on the checkout page.
Information entered in the form on the checkout page is used by other aspects of the checkout process, you should not bypass the checkout page.