I asked by email...

Once registered, is it not possible to add items with this new system, like you could with the old system?
And got this reply...

That’s in version 2 so for this year, just call next week and admin can add the items for you.
So if you are attending and you forgot something or want to add on something extra later (like the Custom class toy or an extra boxsets/loosesets, or had second thought about a class or tour), you have to ring up their office during their office hours (our midnight to about our 8am - Eastern Australia Time).
0011-1-817-448-9863

So next year we will probably be going through the same hassles then, as their planned "version 2" is to allow people to add on items themselves, and they won't work on it until it is time to start registration next year, and then all the bugs will delay it for weeks again.