I'm trying to understand what is the difference between 'Payment API' and 'Extended Payment API' - where can I get this information?
The main difference is that the Extended Payment API is based on building a product catalog and you are required to define your products and contracts before you'll start selling your products.
The Payment API gives you faster results, but your tools are basic compared to the Extended option.
It all depends on what you need for your business.
Another thing - with Payment API you can use JSON or XML scripts, while Extended API allows XMLs only.
Here are some links to the BlueSnap Documentation site you can use: