Is your iOS store app displaying an online web page, in a web browser?
If Yes, then PayPal has payment buttons that you can integrate into your web pages, that are linked
to your PayPal account.
https://www.paypal.com/webapps/mpp/merchant
I cant show you examples, you have to sign up for a PayPal account, and then read there developer pages,
that have tools and payment buttons that you integrate into your web pages HTML code.
If your iOS store app is not based on a web server, but built into the device, then you will have to do the
same as above, but use the PayPal business tools designed for mobile devices.
https://www.paypal-marketing.co.uk/mobile/index.htm
Again I cant show you examples, but the last time I setup a website for a freind a long time back, I obtained
the PayPal payment buttons and tools from PayPal's developer pages, and dragged them into the web
page designer application, and the code for the buttons is automatically added to the HTML code.
But the first stage is to open a PayPal account, and then you gain access to these payment tools.
Hope this is more help.
Regards Mark