I can't answer if they'll fix the pricing for you (that'll have to be Jim), but a couple of things...
MusicMatch is not J River, so that might be part of your problem.
MusicMatch Jukebox was a completely separate competing application from way back in the day (circa 2001, I'd guess, but I'm remembering vaguely) from a now-defunct company. It eventually was purchased by Yahoo and rebranded as Y!Music Jukebox, and then later discontinued.
The way the upgrade pricing works for MC is this:
1. You must actually INSTALL your old version, if it isn't already installed, and get it fully licensed (it can't be in trial mode, it must be "registered").
2. Then install the new version.
3. Start the purchase process from within the new version.
4. You'll get the upgrade licensing.
If when you were trying to set up Media Jukebox 8, you were trying to use license/email information from MusicMatch, that would never work, because that's a different product.
J River Media Center used to be called J River Media Jukebox with older versions, but that was the "original" name (from version 1.0).