I fussed with this problem for a couple hours and had success with the following workaround. Keep in mind I am an iPhone user and make purchases via mobile on the itunes store which I think is related to this issue.
Backstory:
I was attempting to purchase music from the iTunes store and upon the review of billing information it provides the ominous message "Please contact itunes support to complete this transaction" which people are seemingly having no luck with.
I believe (from what I have read) this is an issue with multiple devices sharing an itunes account.
To remedy, I went into the iPhone Settings -> Store -> View Apple ID
Sign in as requested after choosing "Continue" and then tap your "payment info" box. Re-enter the "Required" Security Code field along with verifying the remaining fields. Choose "Done" and then login to your itunes account on the problematic device and retry.
I was able to make this work with two separate itunes accounts with two different accounts so I dont think it was a fluke.
Good luck!!