I use it every month. I'm annoyed it isn't fixed. That said, it forces me to edit each email and I tend to say something meaningful to my key clients.
Here is my routine:
Use BillFlow Manager to create Invoices.
Open the first email in the Outbox folder.
Tab tab tab "inv" - to assign it my “INV” Classification Code.
Tab tab tab "doe" tab - to QuickPick the Matter Name.
Click in the Message field.
Type "John," - and maybe briefly note the wonderful benefits of what we did in the last month. My standard email message that is set in the Billing Preferences appears in the email.
Save and Close - DO NOT PRESS SEND. That could send all the other, unedited emails.
Open the next email.
Go to Step 3 and repeat.
The process is tedious, but much less tedious than printing invoices and envelopes and stuffing and stamping them! It is also much less tedious than manually sending individual emails with invoice attachments.