Songs Purchased from Ipod touch itunes store disappear...

Hi everyone, I purchased a song on the ipod touch recently. After purchasing the song I listened to it a couple times, and everything was normal. Then I plugged in the touch to my MB pro and was letting it synch and do its usual. After it finished, I noticed that my purchased playlist was empty. I looked in my ipod songs library and the song was no longer there either. The Problem was not with the synch, because I have my ipod set to manually synch music and playlists, only auto-synching photos and callenders. So I honestly can not figure out the problem.

This has happened 2 more times since, with the same song repurchased, and a new song, giving me a total of 3$ spent and no songs. And itunes wont even let me re-download the song, it makes me pay full price AGAIN for the song. Any ideas? (The download purchased songs to itunes menu option does nothing, as does the Store, Check for purchases option, because according to the ipod there are no purchased songs)

*By the way I just restored the ipod to factory defaults, hoping that it would use one of the backups of the itouch that itunes seems so fond of making me do. However, after wiping the ipod, I selected the backup to "re-fill" it with, and itunes told me that the backup was corrupt! So I am completely clueless. I have my apple store receipt saved so I may just bring that and the ipod to the apple store and see if they can refund me or allow me to download the songs without paying a third time...