Hi there, @cathal-alpaca-as. 

Let me route you to the right support group that can help you best with mapping the invoice custom fields you've created in QuickBooks Online (QBO) to Hubspot.

Since this concerns the syncing feature in Hubspot, it's best to consult them about how to resolve this issue. You can reach out to them through this link: Hubspot Help Center. 

As a future reference, let me add this article about customizing your sales forms in QBO: Customise invoices, estimates, and sales receipts in QuickBooks Online.

We're always here in the Community should you have questions about custom fields or managing invoices in QBO. We're committed to offering ongoing support. Take care.