Hey Dan! Thanks for taking the time to write in!
You can not delete any invoices that have been sent out of SombreroPAY but you can mark them as ignored. You can do this by taking the following steps:
1) Sign in to your SombreroPAY account.
2) Click on the My Receivables tab.
3) Click on View Invoice by the invoice you want to edit
4) In the Transactions area, click on the "View Invoice" link next to the account you want to mark as ignored.
5) At the bottom of the page, click on the "Mark as Ignored" button.

This will update your unpaid balance for the invoice.