For PayPal Processing you must have a PayPal Pro account, this involves percentage fees just like a merchant account and monthly fees that are higher than a USAePay gateway account.
You can have multiple PayPal configurations. Multiple API signatures can be associated with your API username and password.
The User Name is required to post messages to the PayPal . This should not be the same username as your PayPal account login, but rather the name of the API service you've created in your account. If you allow PayPal to auto-generate the name, it is usually the first part of your email address plus "_api1." plus the last part of your email address. For instance, a PayPal Id of firstname.lastname@example.org would generate a user name test_api1.teststore.com.
The Password is required to post messages to the PayPal . Do not confuse this property with your PayPal password. This password must match the API you create from inside your PayPal account.
The Signature is a PayPal-generated unique digital signature (a line of text, or hash) that you copy from PayPal's website and include in API calls made by this class. Your digital signature, your API username, and your API password all together are called three-token authentication. Multiple API signatures can be associated with your API username and password. For security, every time you view your API credential/signature on https://www.paypal.com, a new signature is generated. In your API calls, you can use any of the signatures that PayPal generates for you. All the signatures are equally valid until you remove your credentials by clicking Remove on the View API Signature page.
You need to enter a pin number to access the credit card setup parameters, all credit card functions that require a pin share the same pin. Initially the pin is set to 0000 (4 zeros), you can use the Edit Pin button that is available in several places to change this pin and you should definitely do this.