If typos and other accidental errors end up being a problem, you could use a different email for proofing or demos. The proofing email wouldn't count against the 5 invoices, and would include a prominent watermark or something. Just a thought.
I wanted to chime in here to backup `flyingyeti's` point. If you slap a giant watermark on it (doesn't count as a hit against the free 5), then it encourages experimentation and interaction with the platform. I'd think you'd want this, as your users will likely discover errors, problems, and output that doesn't meet their expectations.
