What are you expecting when you say "allow users to determine what to download"? Do you mean help you select a distro? If you want this you might be best looking at a site like distrowatch.com
and looking at the reviews associated with some of the popular distros, then decide for yourself.
As for getting copies of Linux distros in the mail, OSDisc.com
will sell you CDs, DVDs and USB Flash installation disks for various distros.
If I have misunderstood you perhaps you could take a shot at rephrasing your question.