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I will have to be very aware of a) the API Calls, b) the Setup and Configuration costs and c) the security model for InvoiceThis.
Apps
Breadwinner
I haven't yet done a full investigation of this product, but it definitely looks like the No. 1 option for Integrating Salesforce and Xero now.
See the Demo Videos on their site https://breadwinnerhq.com/video-demo/ and I am sure you will find a reason that means that Breadwinner will work for you. And if it doesn't you've got two options - go down to Zapier and deal with the occasional failures and manual intervention or go up to a custom development with Trineo.
OneSaas 
Australian product.
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