Connector or custom integration
Marketplace connectors for QuickBooks Online and Xero cover the common cases well: one store or payment account, standard taxes, one currency. When they fit, use them. We say so on the first call, and it costs you nothing to find out.
A custom integration pays off when the connector posts something your accountant has to fix every month: partial payments, deposits, refunds split across lines, fees that must land in their own account, several entities or currencies, or a system of your own that no connector supports. It also gives you the matching rules in code you own, instead of settings in a tool you rent.