Generating Paypal keys
Note: This manual is for osCommerce v4.
In order to receive the payments via Paypal login to your Paypal account, click on Gear icon and then on Account Settings option.
Next to API access click on Update link.
In NVP/SOAP API integration (Classic) section click on Manage API credentials link.
Copy and paste the API username, password and signature into your administration screen.
In order to refund the payments via Paypal from your administration area login to your Paypal developer account, click on My Apps & Credentials tab and then either create an app by clicking on the corresponding button or click on the required link of the existing app.
Copy and paste the Client ID and Secret into your administration screen.
Note: Make sure you choose the required API Credentials (either sandbox like per the screenshot below or live)