Settings → Integrations → QuickBooks (apps/web/src/app/(authenticated)/settings/(shell)/integrations/quickbooks/) drives OAuth connection state (QBOConnectionContent) and sync preferences (qbo-sync-preferences.tsx). Once connected, sync logic (apps/web/src/lib/quickbooks/) covers invoice push and delivery, customer sync, payment sync and reconciliation, and invoice/payment repair. An invoice generated before QuickBooks is connected can't sync; see Troubleshooting for the "QuickBooks integration is not connected" error thrown from invoice PDF generation.

TODO(owner, likely integrations): write the full guide covering what syncs automatically versus on demand, how invoice-matching/ reconciles existing QuickBooks records against solarOS invoices, and what hold-reasons.ts surfaces when a sync is blocked.