Test URLs
Firewall
How to configure firewalls - Do you have a strictly configured environment? Take a look here to get proper settings for your firewall/web services. Link to faq
Merchant Portal
For the Merchant Portal go to:
https://merchantportal.integration.resurs.com/login
You can see the generated invoice for payment method INVOICE in Merchant Portal, since the invoice is not sent by mail in the test-environment.
MAPI / Merchant API
See MAPI documentation here for more information.