1. Turn billable expenses on. Go to the gear icon, then Account and settings, then the Expenses tab. Turn on Make expenses and items billable. While you're there, you can set a default markup percentage and choose whether billable costs should track to a separate Billable Expense Income account.
2. Mark the expense billable when you enter it. On an expense, check, or bill, assign the Customer it's for, and check the Billable box next to the line. Add a markup here too if this particular charge needs a different rate than your default.
3. Save the expense as usual. It posts to your books like any other expense; the only difference is it's now waiting to be passed through to that customer's next invoice.
4. Add it to the customer's invoice. When you create, or open, an invoice for that customer, QuickBooks shows the billable expense under Add to invoice on the right side of the screen. Click Add to pull it onto the invoice as its own line.
5. Confirm it's off the pending list. Once it's added to an invoice and saved, that expense drops off the billable queue for that customer, so nobody accidentally bills it a second time.