Hi James678, thanks for posting on the Community
There isn't a feature to add an expense/billable expense to an invoice in QuickBooks Self-Employed.
Instead, you would enter separate lines on the invoice (with a minus symbol beside the amount if you want this to reduce the overall invoice total).
Please note, any income received from the invoice should be entered separately on the Transactions page and tagged as Business income. Likewise, any expenses should also be entered on the Transaction page and tagged with the relevant category, so that your tax can be estimated accurately.
Thanks for reaching Intuit support. We're just a post away if you have further questions.