Is it possible to start an invoice, sort of as a draft, and not enter a due date?

Your workflow is correct, Accounting41.

In QuickBooks Online, you'll have to manually change the due date if you'll base it on the sent date, not on the transaction date. You can create an internal custom field, then add notes so you will not forget any details about the invoice. 

Here's how:

1. Click the Gear icon, then choose Account and Settings.
2. Go to the Sales tab.
3. Click the Pencil icon in the Sales form content row.
4. Under Custom fields, enter a desired name.
5. Put a check mark in the Internal box.
6. Click Save, then Done. 

I've attached screenshots for your reference.

If you still need my help, just leave a comment below. I'll get back to you as soon as I can. 

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@accounting41 wrote:
Sometimes I start an invoice for a client to hold all their charges until the project is complete. It mostly holds my service/product charges, but it will occasionally hold expenses as well.
What I've found is the invoice will automatically generate a due date based on when I created the invoice. But I want the due date to be 30 days from when I actually send it. I know I can change the due date later, but I'd rather not as its easy to forget. Perhaps my workflow should be different?

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It is better to use  an estimate, an invoice is a final sale in QB an an estimate is non posting.
List charges as they occur on the estimate and when ready copy to invoice, that avoids that issue.