When you invoice a customer for a billable expense, does it credit the original cost account?

Welcome to the Community space, Jennifer.

Yes, you're right. Please know that in QuickBooks Online (QBO), when you invoice a customer for a billable expense, it uses or credits the original cost you added to the item's information. Feel free to check this article to learn more about how billable expenses work: Enter billable expenses in QuickBooks Online.

In addition, you can visit these articles to help you manage customer's payments and review all transactions you entered inside the program:

• Record invoice payments in QuickBooks Online.
• Run reports in QuickBooks Online.

You can mention me in the comment section if you need further assistance managing invoices or have any additional questions related to the program. Feel free to visit us here in the Comment space. Stay safe.

@JenniferFreed 

All the billable expense option does is earmark the expense so that you can add it to a customer's invoice.  You assign the Product/Service of your choosing to the amount of the billable expense, but it has no impact on how the expense was originally recorded.