QuickBooks Online allows mapping to 3 custom fields.
  1. In Presidio, open TOOLS > SETUP > SYSTEM SETTINGS the click the Invoicing tab.  There is a section allowing you to map to 3 custom fields in QBO in the order you want them to appear (Job #, Job Description and PO#).
  2. In QBO, click the Gear Icon, then choose Account and Settings, choose Sales on the left margin, then under Sales Form Content, click Custom Fields. There will be a section to enter a description for the 3 fields.  You do not need to use all 3.