To add Fat Zebra as an invoice payment option for Xero Invoices the following steps should be taken within your Xero account:

Click on Settings and then General Settings
Click on Invoice Settings
Click the Payment Services button, and then click Add Payment Service, and then choose Custom Payment URL
Name the service Fat Zebra, and set the service URL to the following, replacing username with your Fat Zebra merchant username: https://paynow.pmnts.io/invoice/username/[INVOICENUMBER]/[CURRENCY]/[AMOUNTDUE]
Click Save
Go back to the Invoice Settings area, and click the Options button for the invoice template you wish to use this service for, and choose Edit
In the Payment Services drop down select the new Fat Zebra service you have created, and then click Save

Once this is completed invoices which are sent via Xero will include a link to view the invoice online - the page hosting the invoice will also include a link to pay the invoice, which will redirect to Fat Zebra hosted payment page.