That is exactly what is happening.
If I 'include' (as opposed to require_once) the definition, it throws the error:
Fatal error: Cannot redeclare WA_eCart_WeightConversion() (previously declared in /home/accountname/public_html/WA_eCart/WA_eCart_Definition_PHP.php:514) in /home/accountname/public_html/WA_eCart/WA_eCart_Definition_PHP.php on line 605
and never even gets to the $eCart1->GetContent(); line
I cant be the only person though that uses eCart to send an email receipt, and uses an external file to do it?
This is something that I was really looking forward to in UE4, as it makes it MUCH easier to edit and customize the email receipts.
Definitely a strange error!