As you may have noticed ZB has 2 types of PayPal Processing:
Express CheckOut (3ºparty hosted)
Credit Card (Self-Hosted)
This is the traditional Payment integration. ZB redirects you to PayPal payment web page and every action is inside Paypal website. After complete successful transaction PayPal redirects for ZB Invoice and set as Paid. Simple, just set you paypal registration email and ready to go! Take a look:
Self-Hosted Credit Card. Need off line payment? Want to process payment automatic every time a new invoice is created? This is the way.