I like to send invoices with notes and edit the details. The latest WHMCS update doesn’t give you this option, so here is a work around.
Duplicate the original system generated invoice to create a draft copy. Edit the new copy and then delete the original as follows.
Go to your unpaid invoices
Select the invoice you wish to duplicate and at the bottom select duplicate.
Go back to invoices > drafts and edit the invoice.
Use the next available invoice number, managed the edits.
Publish the new invoice.
Click Publish and either select send or download and send manually.
To cancel the previous invoice, go to that invoice and click Mark Cancelled under the invoice status.