According to the BT web site I think if you order the HD Extra bolt-on for £3 they will also throw in the BT Sport HD for free if you are a subscriber to the BT Sport pack. So there should be no need to order BT Sport HD separately if BT set up your account entitlements correctly.
A selection of TV channels in high definition for stunning picture quality and glorious detail. If you add HD Extra with BT Sport you'll also get BT Sport in HD at no extra cost.
Did you already have BT Sport set up before ordering the HD Extra package or did you order BT Sport afterwards?
Though just to confuse things (BT like to do this) the current BT Tariff Guide states the following,
"If you take the BT Sport HD Pack there will be no charge for BT Sport HD if you have HD Extra."
What exactly does it mean by "take the BT Sport HD Pack"? Does that mean you have to order it separately, which is what you tried and found didn't work? When you tried to order it online did it actually state that it was free because you already had HD Extra?
I don't know how things work on the BT Youview+ box, but on the BT Vision+ box you can see your multicast channel entitlements via the settings menu and along with others there is one called 'HD Extra' and another called 'BT Sport HD' so it's pretty obvious if the account has been set up correctly for both or not.
I can have this investigated for you to see if we can get this added on for you.
Send me an email using the contact the mods link in my profile.