How to add Billable Expenses to an existing already created Invoice

Open an existing invoice. Look for an arrow pointing left at the top right corner (right to BALANCE DUE). Click that arrow You will find the Billable Expense sitting there and click Add. You will notice line item from the existing invoice and added a billable expense line item. Delete line item from an existing invoice. Now, you have linked billable expenses to an invoice and the total invoice amount remain the same.

Link Billable expenses to an existing invoice