e-commerce for soft-goods downloads (purchases)

[Shorty]

hi...

i know there must be some good software out there, but where?

for musicians who sell music or artists who sell photography or for
researcher who sell documents... is there any good software or services that
will facilitate the ordering, paying-for, and downloading of soft-goods from
the internet?

i'm trying to design a site for a fellow who is selling a single research
document and simply needs a process whereby payment authorization leads to a
download window with a "secret" URL and a small window of time to download.

any comments appreciated...

shorty