Is he maybe just a really literal person and took the forwarded email with instructions "send THIS to them" as "send this email to them", but then he knows his work responsibility is to send the product, hence why he sent both the product and the email?
Do you have an SOP or exact steps to follow written down in a very clear, concise manner (yes, even something as simple as "after employee A sends the email, employee A will notify employee B to send out the product by forwarding the customer email to employee B"; I'm talking reaaaallly literal steps here)?
Can you give other examples of times he's messed up in a similar way?
Does he have poor reading comprehension? ADHD?
I train employees a lot in my current job and jobs prior and I've learned that everyone interprets things differently and learns at their own pace. Most times you can adapt to their style, but sometimes people are just mentally out to lunch 24/7 and not fit for the job.