I have to correct you.
Schlager is extremly relevant here with pretty high sales and it is a form of pop music (take a look at german Wikipedia:
http://de.m.wikipedia.org/wiki/Schlager).
Though highest selling female Schlager singer is Andrea Berg, Helene Fischer is just No.2.
But as I don't listen to Schlager, I can't recommend you any songs, I'm sorry. xD
And btw. Volksmusik actually has the highest percentage of music sales in Germany and Schlager is normally included.