I’ve had a system for several years that wasn’t perfect but it does the trick. I use Spark but for this process, I sometimes just did it using Apple Mail.
In Automator, I created a process that opened up Apple Mail, created 5 different emails which included the recipient(s) and the body of the email. All I then had to manually do was attach a unique invoice to each of the emails and send it.
Sometimes, I would have Automator run the script and I would save the email as a draft. Then, I would open up Spark and it would be sitting in the drafts folder and attach it there. (Don’t ask why I sometimes did this.)
I recently upgraded to Ventura and it’s caused two problems.
If I attach the invoice in Apple Mail, it attaches it as an inline file which I don’t want. Not sure if this always was this way.
If I attach in Spark, when I open up the draft in Spark, it renders the email as if I had forwarded the email to Spark with the body text indented.
I’m open to other solutions. Don’t have to use Automator.