What is the difference between the simple and the extended PayPal API?

A. Simple PayPal API:

1. It requires only your PayPal address in the Billing > Settings > tab “Merchant & PayPal”.
2. You can receive almost all kinds of payments (for automated and manual agreement-based invoices, manual invoices outside agreements created with button “Add Miscellaneous”, one time fee products) with the exception of payments for recurring billing products (sold in payment installments), which require the extended PayPal API, meaning the PayPal API credentials need to be entered (correctly) in the “Recurring Billing” tab of the Billing Settings.
3. You cannot refund an invoice (recurring or otherwise) with the simple PayPal API; you need the extended PayPal API for refunding all types of invoices.

B. Extended PayPal API:

• You can receive all types of payments, and you can refund any type of invoice directly from the Console, but – importantly – you cannot mark an invoice as refunded if you refunded it directly from your PayPal account.