Modification of the presentation of page appearance, page navigation flow, and other aspects of the user experience of an online shopper navigating the virtual shopping cart and checkout pages of an online store is achieved without compromising the security of checkout processing. The modification may include customization, and various configuration and customization tools may be employed. The online shopper experiences continuity between unsecured-connection pages presenting items for purchase and secured-connection pages presented for checkout processing. In one embodiment, page navigation flow is modified using continuations implemented using a relatively small stack to save information relating to the flow. In another embodiment, a page designer enables a dynamic version of a page specification program to be generated, such as by way of a page customization interface. Partial evaluation is employed to generate a version of the program that is executed at request time.