How can I send a supplier credit invoice via email?

Thanks for visiting the Community, @tolga. At the moment, you can only email any sales forms to customers straight from the system but not the expense forms such as the Supplier Credit. 

As a workaround, I'd recommend taking a screenshot or photo of the supplier credit, then email it manually to your vendor outside QuickBooks.

I can see how essential this functionality for you and your business. I'll take note of your feedback and share this preference with our engineers for future consideration. 

Here's a great resource for your future reference: How do I handle supplier credits and refunds?. It helps you learn about how to pay bills using supplier credits.  

Feel free to visit again if you have additional questions. We're always delighted to help you some more. Keep safe.