Spreedly has just made support for JSONP publicly available for adding payment methods. While we have had support for CORS for some time now, the addition of JSONP improves options for developers. The transparent redirect is still a great places to get started for adding payment methods with Spreedly. 

It is a simple form that doesn't require any custom client side code. However your needs may be more advanced. For example, you may have a single page app that needs to remain active so you don't want a page refresh. CORS and JSONP can give you that flexibility by using JavaScript's asynchronous request capabilities. We've also updated our documentation for CORS to use JSON as the data type rather than XML since it is the natural choice for web and mobile apps. 

Rest assured that XML will remain fully supported. If you choose to do mobile development in a native language with a networking library, you can choose to directly add payment methods. It is an endpoint that is friendly to JSON that you can work with directly for creating payment method tokens. With all of these options to choose from, it can take a little bit of analysis to choose which one is best for your needs. 

Our adding payment methods page outlines the different options and assists you in making a decision.