Finally got the damn thing through after trying about 20 times with 2 different computers and 3 internet browsers. What an ordeal. At least that's it for the SHF releases that are coming out this year.
There this trick too, that you add the item to your cart once, and it fails afterwards when the order doesn't go through. Because it's limited to one per customer, it doesn't let you add it again when there's already one in your cart from the last time round where it failed.
At least I don't have to do the same for the 2 S.I.C. Showa riders coming up, since the pre-orders don't go out as ridiculously quickly as SHFs do.