Xero
Invoices, both ways. Your invoices go into Xero on their own, the customer is created there if they are new, and when they pay, Xero tells Sign OS and the job is marked paid. It includes a one-off contact import, not continuous contact syncing: you can import your existing Xero customers, suppliers and business details directly, choosing what comes across. After that, editing a contact here does not change it in Xero.