You can pay directly from the app by going to Site Settings (top right button) and "Upgrade or Renew".
To see all the payment methods Apple offers in your region visit:
Payment methods that you can use with your Apple ID
If you are having problems paying in the Apple App Store, there are other solutions available:
Login into your SimDif site with a web browser, on a phone or a computer:
Go to "Upgrade or Renew" and you will see the alternative payment methods we offer.