To lower your total costs, use a third-party processor (like Authorize.net) integrated via a Xero app. Xero does not charge extra for this integration, but the gateway will.
Here is a general breakdown of how they structure their tiers (though names and specifics can vary by region): xero fees work
Xero’s fees pay for cloud hosting, continuous updates, security, integrations, and features that automate bookkeeping tasks: To lower your total costs, use a third-party