I'll share details about how QuickBooks works in adding bundled items on transactions, @CBS100. 

The quantity works as designed since the items you added from an estimate to an invoice are bundled. That's why it shows one, and you're unable to change it to zero. This is also to keep track of the actual quantity of the items in your inventory.

However, if you still prefer to change the bundled items' quantity, we can change it directly from the estimate. Please know that if the items are entered in zero quantity, no amount will be calculated or converted to the invoice. Here's how you can edit the estimate:

1. Go to the Sales menu and select the Estimate tab.
2. Locate the estimate you want to make changes to.
3. Under the Action column, click the Edit option.
4. Then, enter your desired quantity in its respective column.
5. Once done, click Save and Close.

Furthermore, you can check out this article to see how you can record an invoice payment once your client pays their statement of charges: Record an invoice payment. 

Please let me know if you have any more queries about selecting a product or completing any transactions using the application. Help is always available from the Community. Stay safe!