To accept Paypal as a form of payment in your Nexternal Order Management System (OMS), you must first create an account with Paypal to receive funds from your customers.
It is recommended that all merchants use the Paypal Express Checkout option for the PayPal/Nexternal connection. (Settings / Edit Billing Options / PayPal).
The Use Paypal Express Checkout in Online Store option allows customers in the Online Store to log in to Paypal and approve payment upon submitting an order via an overlay window on the Invoice (final checkout) screen. This option thus ensures that customers in the Online Store who choose to pay via Paypal actually remit payment. To use Paypal Express Checkout, you must provide your Paypal API Username, API Password, API Signature, and Merchant ID; which are obtained by logging in to Paypal and clicking My Account/Profile (you may need to request API Credentials if they do not yet exist).
Note: Paypal Express Checkout does not apply when Pending Orders are placed, as payment for a Pending Order is collected only when the order is activated. Similarly, Paypal Express Checkout does not apply when an order is placed by a Subcustomer in a Customer Purchasing Group (even if the order does not require approval), as payment is the responsibility of the Master Customer.
Note: This guide is based upon the Paypal interface as it existed at the time this feature was released (several years ago). Your PayPal interface may look different, but this guide should give you enough information that you can find what you are looking for. If you can't find the information you need within PayPal, please contact PayPal customer support. Once you have that information, follow the steps in the guide to enter the data into your Nexternal OMS at (Settings / Edit Billing Options / PayPal). Be sure to read the ? tooltip next to PayPal for complete details.