Short answer
HubSpot's KB: when you create an invoice, the integration checks it against your filters and, if eligible, syncs it to QuickBooks Online. You can add one-time order-level discounts, but you cannot add invoice-level taxes and fees, because QuickBooks Online does not have the same concept. Failures open in the integration's Failing to sync panel.
1. Where syncs live
Monitor app syncs from Data integration in my demo portal.

2. Common confusion
- Missing invoice: check the sync filters first.
- Tax line missing: invoice-level taxes are blocked with this integration.
3. Related
Invoices stopped syncing and Stripe data sync.