On another note...

The first line of the certification that the taxpayers signs says

"Under penalties of perjury, I declare that I have examined a copy of the income tax return (original or amended) I am now authorizing, and to the best of
my knowledge and belief, it is true, correct, and complete."

It sounds by your question like you are not providing a copy of the return to your client until after they have signed the 8879 and the return has been e-filed.

Your system may be ok...just so much different than what I have seen in the practices I have been around, where the client was provided a copy of the return prior to, or concurrent with the 8879 signing.

Perhaps I misunderstood the wording in your question.