This chapter has focused on Google Checkout as a payment service provider, but numerous other providers exist. Amazon offers a Checkout service as well as a Flexible Payments Service. We will detail the FPS technology later, in the chapter on selling digital goods.

The Amazon Checkout service functions very similarly to Google Checkout. For example, constructing a shopping cart and submitting it to the Amazon Checkout service involves creating an XML document, rendering it with our shopping cart content information, signing, and submitting it to the appropriate HTTP POST URL. The views and utility functions written in this chapter could easily be adapted for Amazon's service. In some cases the only difference will be in the format and tags used for the XML file.

PayPal is another popular payment vendor. Their payment workflow tends to be different than Google or Amazon, but this depends entirely on the API you are writing against. As of this writing, PayPal offers a dozen or so payment APIs, each with different objectives or target applications.

The most similar PayPal API to what we've discussed in this chapter is their Express Checkout. The simplest integration with this API does not involve transmitting the shopping cart contents. An order details feature is available for use with Express Checkout that provides a similar user experience to the checkout tools we've already discussed. However, PayPal APIs do not rely on an XML document for transmission of this information; they instead use a Name-Value Pair (NVP) interface. These are just encoded HTTP GET parameters added to special endpoint URLs. Alternatively, PayPal also offers a SOAP interface.