QuickBooks Online has a function that helps us link the billable expenses to your customer invoices. If you haven't turn on this feature, then here's how to do it:
After that, you should be able to create billable expenses. Thus, allowing you to link them to the invoices. You can check out this article for more information: How Do I Enter Billable Expenses by Job?.
Meanwhile, we're unable to directly add the billable expense receipts as attachments to the invoices. As a workaround, you can take a screenshot of the billable expense receipt and save it on your computer. Then, open the invoice and attach the screenshot.
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If you're referring to something else, any additional information would much appreciated. Thanks in advance!