The fakturan.nu API is a REST API with JSON as data format for both input and output. Here's how to get started with development:
1. Setup a sandbox account
Register a new account in our sandbox environment. API-keys are created on the settings/api_keys page. You should create one key per application. Any invoices sent in the sandbox environment will be sent to the e-mail address you chose at signup (it can be changed later from the settings/user page). Letter invoices are not sent at all from the sandbox.
2. Choose a client library (or roll your own)
See the libraries page for available client libraries. Each library comes with it's own documentation and example code. If you want to roll your own, or if we don't have an official library for your language of choice, have a look at the curl examples or the documentation.
3. Get API-keys for production
Just go to the settings/api_keys page on your real account to create a production API-key.
Read more about how to use api-key-ID and password with http basic auth here.