Hi richard_dns,
Welcome to Community! QuickBooks Online has features you need to reach your daily goals. Converting estimates into invoice can be done effortlessly. I'll be happy to provide more insight here!
In QuickBooks creating an estimate is a great way to present details of your work to your customer. You can then convert it to an invoice so it's not entered twice. They can be attached to an existing invoice as well. With regards to having the same capability with Zapier, I recommend reaching out to them directly for their expertise on the functionality for creating invoices.
Feel free to reach back out with other questions. We'd be glad to assist!