We already explained it here:
http://www.fsdreamteam.com/forum/index.php/topic,24166.0.htmlBy default ( I think this is listed in your purchase receipt ), your download link is valid for a month or so. Normally, all our products had downloads always freely available, because there were Trial version but, in this case, since there's no Trial, the download link is not public, so it use Digital River's rules.
That's why we chose, for this product, to ALLOW the Download protection service, which means Digital River will keep your link active for 2 more years. That's the maximum amount we can set, and you are not forced in any way to buy that service. In fact, we suggest not to, because we'll surely allow some way to download the whole scenery from our Live Updater, which now CAN verify your purchase.
This feature is not there yet, so we suggest making a backup of the installer for the time being, but I can assure you we'll find a solution that will allow you to redownload the whole scenery at any time, with no expiration.