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 Google offers in your region visit:
Accepted payment methods on Google Play
If you are having problems paying in the Google Play 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.