Xcode iPhone Development - ecommerce app

I've read Apple don't allow you to create your own e-commerce iPhone app and integrate into Paypal, some payment gateway and you should only use/ go through the app store using the storekit. How do other companies such as Amazon do it?

Any links to any extra reading would be great,

Thanks in advance.

You must use storekit if your products are all virtual. But if the products are physical (the case with Amazon) then you must not use in app purchase (storekit) and must process payment on your own. Check In App Purchase Programming Guide for the details.