A data field for the customer purchase order is standard on invoices in SOS Inventory—but not in QuickBooks Online. To get this information to sync from SOS Inventory to QuickBooks, create a customer purchase order field as a custom field for invoices in QuickBooks Online.
Please note that QuickBooks Online syncs only the first three custom fields you create for sales and the first three for purchases. They must be created as Text and Number fields and must be transaction-based custom fields in QuickBooks Online.
To fix this situation, do the following:
Now, any future invoices created or edited in SOS Inventory will sync to QuickBooks Online with the Customer PO included.
Future invoices created in SOS Inventory will now populate the Customer PO onto the invoice in QuickBooks Online.