My accountant needs an Excel file for billing

You can give your accountant an account and access only to Billing or you can do this and send him/her the file.

1. Go to Billing
2. Click the Reports tab
3. Click on "Invoice Breakdown"
4. Select a Report Period (there's a custom date range picker)
5. Click "Generate Report"
6. Click on the Export button besides the print button

That will export in Excel format.