Your link must follow the Plain Button style, as specified in the Human Interface Guidelines. It may not be enclosed in a shape that uses a contrasting background fill. The background surrounding the text must match the background of your app’s page. The link out icon provided by Apple must be displayed directly to the right of your website URL. The icon size must visually match the size of the text.
Apple’s new guidance on linking to external sites for payment
Forcing developers into iOS 7 style flat design, so that users will tap the ”real” buttons for in-app purchase. What an endorsement.