Xero Bridge by Parex

This app has been a huge asset to our business, both in creating Shopify sales invoices within Xero and in syncing inventory items/quantities between the two platforms. Doing it all by hand was unsustainable, and this app made it a breeze.
Additionally, Mahek from the Parex team was incredibly helpful and thorough in assisting me with some issues/settings that needed to be tweaked shortly after implementing this app. If this app sounds like it'll accomplish what you need it to, I'd recommend giving it a shot - you won't be disappointed with the customer service.