Is there a way to send an invoice with the 8879 forms and tax return that must be paid before the client can download the return?

You must give the copy of the return to the client in order for the client to sign the 8879. I generally bill client before sending 8879, and copy of return, but be aware you are required by IRS to give client copy of return to sign 8879. 

Yeah, kinda hard for a client to review the preparers work if all they see is an 8879.  That sounds more like a kidnapping  -----"if you ever want to see your return alive again, give me money".  I bill my clients when they receive the return.  Some pay on the spot.  Some pay a few days later.  And of course, some take what seems like decades, but they do pay.  I probably shouldn't write a book on how to collect money, but I somehow manage to keep the doors open and keep my clients happy so I guess it works for me.

In 40+ years I could count on one hand the number of individual tax clients who did not pay me.  So to me your concern would not be an issue.  Then again, if I was concerned I wasn't going to get paid, I would not have them as a client.