if one customer shows up, and she buys one song, that's $1.29, if she doesn't buy anymore, then that's the charge.
the first song is always at a full price, and after that, to encourage buying, the next ones are just 99 cents, let's see how it goes after a few songs purchases.
1 song.........................$1.29
2 songs.......................$1.29 + 0.99(2)
3 songs.......................$1.29 + 0.99(3)
4 songs.......................$1.29 + 0.99(4)
c songs.......................$1.29 + 0.99(c)
s(c) = 1.29 + 0.99c.
