The invoice number sent to Authorize.net is specifically intended to include a random set of letters after the main "number" for the purpose of ensuring uniqueness. This prevents the (relatively rare, but still real) possibility of two customers placing an order at exactly the same moment on your store and sending the same anticipated invoice number to Authorize.net for processing. Authorize.net won't accept two transactions of the same number. And, since Zen Cart v1.xx doesn't actually assign an order number until AFTER the payment is completed, it must guess the next number. It's that guess that creates the possibility of duplicates. Hence the random letters added for uniqueness.
As far as Authorize.net is concerned, the invoice number DOES include those random letters. As such it's very appropriate for them to include those letters in the email.