Customers can't access their invoices as it's showing as 'invoice unavailable'. What can I do to fix this?

Let's fix the error your customers are having to access their invoices successfully, @david296.

Can you tell me more about where they're trying to access the invoices? We currently have an ongoing issue when your customers are trying to open the invoice sent to them using an iPhone. Our Product Engineering team is already aware of this and is now working on a fix. 

To constantly receive a status update, I suggest reaching out to our Product Support Team. Then, let them add you to the email list for Investigation No. 61615.

You can reach our Customer Support for QuickBooks Online (QBO) by going to the Help icon at the top right of the account. Follow the steps below: 

1. Go to the Help icon to connect to a live support agent. 
2. Click on the Contact Us button and provide some details about this concern.
3. Provide us some information about your concern in the What can we help you with? box and click on Continue.
4. You'll now be provided options on how to connect to our Customer Support.

You can check this article for more information about the different types of support we offer and their availability: QuickBooks Online Support.

However, if they're trying to access their invoices using an android device or through the web, you can let them right-click the Review and pay button in the invoice email and copy the URL. Open a web browser and paste the link into the address bar.

If the link still doesn't open, try other supported browsers (Firefox, Google Chrome, Safari, Microsoft Edge).

Once you've received their payment, follow this article on how to record payment or partial payment for an invoice: Record invoice payments in QuickBooks Online. 

Let me know how this goes and leave a reply below if you need further assistance in processing invoice payments. I'll be around to help. Take care always.