return redirect()->back();

Adding exit; didn't make any difference.
In reading some more - is it possible that PHP is waiting for a return from the PHP mail function?
Maybe I need to find a way to send the email asynchronously?
I found this code to do that in another search - exec("php mailuser.php > /dev/null 2>&1 &"); - but I am hesitant to use it after reading comments about its security and other issues regarding long-running threads...

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I tried doing this instead of the return:

$this->index();

That did work to display the shopping cart - and did send the email, so I'll just use this and quit trying to figure out why the return from a function didn't work.