Here's how to automatically create a client, contact, and project in Harvest whenever an opportunity is marked as won in Copper. It then outputs all of that data in a very organized matter into a Slack notification.
The entire article with images can be read here.
We have since added to this Zap to also have it create that customer in QuickBooks Online with all of the data. In addition, we made it so it creates a Copper custom field within the Company entitled "Harvest Invoices URL" which contains a unique URL that links directly to that client's Harvest Invoices (paid/unpaid). This requires the use of custom Webhooks so it is not as straightforward.
With this in place, we can click a link from the company page to immediately see that client's paid/unpaid invoices and make any modifications we need to. It's as close as we can currently get to a "native Harvest invoices" integration.