A: Most organisations dont really want to link every single payment/donation with their finance system. A report can be run and reconciled with the total banked (from the payment gateway) and then journal the amounts in. This report is often summarised by payment type (Credit Card, Cash, Cheque) and by Campaign (or a GL Code text field on Campaigns). There is a report that comes with Payments2Us called Daily Banking which reports this information.
The cost of the integration software between Salesforce and Xero is can be rather high, so the manual journals is quick and cost effective if you are wanting to reduce costs.