After the confirm order page, and before the checkout, the browser of the client will be redirected to the Paypal Payment Service. After the client has payed, its browser is redirected to the "order overview" page. The ordered products are not directly visible in the administration interface. The Paypal Service will contact the Exponential shop first. This service will notify the shop of the payment. Hereafter the shop will test the authentication of the notification.

The following figure illustrates the communication between the client browser, webserver which hosts the shop, and the paypal payment service:

The callback of the Paypal service to the shop requires the webserver to be available via the internet. To check whether the callback is valid, the shop will query the payment service. Therefore the webserver must be able to request a webpage of Paypal.

To test the shop you should have a Paypal merchant and customer test-account. If you do not have these test accounts, you can register them (for free) at: http://developer.paypal.com