There has to be some kind of glitch, as I was able to select EC for free for a flight in two weeks (AMS-EWR). I'm gold so I shouldn't be getting it for free..
As I made my seat selection and wasn't asked to pay I logged out and logged back in and I seem to still have that seat, even though if I try to select other EC seats the system will say I have to pay. But still, I got that one seat for free...

I suggest you call the Delta Medallion Desk ( since you are ST Elite ) and just ask them to do it, there should be no problem. Otherwise try twitter...