I appreciate you for raising your concern here, @Shifa.
There are various reasons why invoices are undelivered from QuickBooks Online (QBO). Let me share this with you and provide the steps to fix it.
An invoice fails to deliver if the email address is incorrect or there's a temporary issue with the recipient's email server.
You can review the customer's email address if it's incorrect. Here's how:
- Go to Sales, then Customers.
- Choose the customer who had the undelivered invoices.
- Click Edit.
- Review the email address and edit if it's incorrect.
- Then, resend the invoice.
Check out this article for more info: Fix undelivered invoices in QuickBooks Online.
If you see no incorrect info in the customer's profile, I recommend contacting the QBO Care Team to help you identify the reason for the issue and provide you with a fix.
In the meantime, I suggest editing the undelivered invoice and clicking the Print and Download tab below the form. Then, manually send it to your customers.
Concerning the undelivered invoices in QuickBooks API and Sandbox account, I suggest contacting the Intuit Development Team. They have the complete tools to locate the root cause and provide you with a resolution.
If you want to personalize your invoice form before sending it to your customer, you can read this article to learn how: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Let me know if you have additional questions about sending invoices in QBO by replying to this post. We'll be willing to help. Take care always, and have a good day.
