It appears that Billable Expenses can only have a NEW invoice and can’t amend an existing invoice.
It you only want to do one invoice you need to allocate the expense to either a Current Asset - Transaction Clearing account or any (paint) related P&L expense account. Then instead of putting the purchase to Billable Expenses, use the Transaction Clearing or the P&L account. Then edit the existing invoice, add line, and use the same account as the purchase. This will contra out any purchase, just as Billable Expenses does.