If your app is an online store, accessed from a web site from the iOS device, then look into setting
up a PayPal account, and then use one of the navigation controls of your app, to link to the PayPal
web site.

I believe that companies like payPal have a developer program, where you can integrate their
payment services into your web site, or web application, these services allow you to install there
payment buttons and logo's into your web pages and apps.

Hope this is some help.

Regards Mark