There is very little in the world that is unique. You have to trust or spend a lot more $$$ or do it yourself

I suggest that you mail to yourself, via postal mail, a copy of your document. Leave it unopened. That represents a time stamp for the work. If it ever went to court, you could say, "Here is the postmark, and the work is inside". If instructed to open it in court, your first position is secure.
Then, when submitting to the fiverr agent, ask them to add the copyright seal, since you are telling him it is copyrighted material.

Can't suggest alternatives but I can strongly suggest you NOT trust anyone on fiverr.