Normally, we generate a thank-you page with a unique URL for each buyer to download their own copy of the product file, so we can expire that URL after so many attempts or hours. However, since you're returning completed project files to your clients, rather than selling a digital product like a software program, ebook or music, you may not be concerned about your buyers sharing their download page with others.
In that case, you could set up a separate product for each client (as I explained in your other thread here), but using our product-specific Redirection feature to send them to their own project's "secret" download page on your site after checkout. With this approach, you wouldn't even need our $18 plan, since you wouldn't be serving the file itself through our download system, just using us to automatically disclose each client's secret download page to them after they have paid you:
http://www.e-junkie.com/ej/help.redirection.htm