I appreciate you joining this thread, Smem. I'll help you fix this case by outlining a solution that saves you time managing your unsent invoices in QuickBooks Online.

Before we dive in, may I know what specific error you encountered while delivering your invoices? This way, I can identify related cases and find more solutions to resolve them.

There are two ways to verify the delivery of your invoice in QuickBooks Online. To begin, you can send a test invoice to yourself. You will need to provide your email address for this process. After saving and sending the invoice, please check your inbox to see if it has arrived. Once you have confirmed receipt, delete the invoice to prevent any duplication in your invoice count.

Here's how to delete it:

1. On the navigation menu, then select Invoices under the Sales tab.
2. Choose the test invoice to open it.
3. Press the More Actions, then select Delete.
4. Click Delete again on the confirmation window.

Moreover, please verify your inbox, spam section, or junk email folder, as messages can sometimes be filtered and stored there.

Another option is to forward yourself a copy of the invoice sent to your clients. Doing this will help you keep a documented record of the invoices they obtain.

Please follow the steps below:

1. Go to the Gear icon.
2. Choose the Accounts and Settings.
3. Navigate to the Messages section from within the Sales tab.
4. Choose Email me a copy at [your email address].
5. Select Save and Done.

If you've already tried the options above, I recommend removing the customer's email from the invoice and re-entering it to ensure accuracy. You can also resend the invoice in an incognito window or through different supported browsers.

If the problem continues, here are additional solutions to fix the undelivered invoice. Refer to this link: 3 solutions for when customers aren't receiving your emails.

Additionally, see this resource that will help your customer pay their invoices via online payments: Receive and process payments in QuickBooks Online with QuickBooks Payments.

If you have more questions as you work through your invoices, please don’t hesitate to reach out. The Community forum is always available to provide support and guidance.