While it is based on an interesting mathematical principle, the attempt to turn it into a magic trick is in my opinion terrible, and nothing that could be used in any kind of real environment. It is far too procedural to be entertaining, and in my opinion, would never produce anything like the audience reaction that the advert suggests.
More than that, without giving away the method, the combination of the names on the postcards, the way the postcards are arranged beforehand, the nature of the instructions, and the precise and exact manner in which they are delivered would make any audience scream "mathematical principle in play here".
If that weren't enough, the instructions are so exact and have to be followed while your back is turned, which means that the spectator could very easily make a mistake. Since you would have no way of knowing they'd got something wrong, there would be no way for you to recover and your big reveal would be totally wrong.
I therefore recommend against getting this effect.
I do however strongly recommend continuing to use this site, as I wrote a more detailed version (including my observations on the method) of this review to Chris Wasshuber after I bought the effect, and as part of his reply he said "I would also like to ask you to share your opinion in a review of the product. This would help others better understand if this is the right product for them or not. Also star-rating your purchases would be a great way to help weed out the good from the bad." This would suggest that Chris is a trustworthy seller who is concerned to make sure that his customers buy things that are right for them, rather than just wanting to sell things to us, and that's a rare thing in this internet world!