Here's an update:  I created a new template as was suggested, and tried sending that via "Send Forms" as the customer (member, we are a chamber of commerce) is flagged to "email later". It failed in the same way.

I then tried emailing directly ... NOT through "send forms" but via the Email link on the invoice screen ... not the "Email later" method.  This one worked just fine and the member received the invoice PDF.

So I believe that there is something wrong with the "email later" option and Send Forms ... something is different between immediately emailing the invoice and emailing it "later". I am going to submit a bug report.