As far as I know, Apple have stopped making the 64 GB iPhone 5S (link)
So I guess what Core/iStore are hoping for in this next shipment are just more 16 and 32 GB 5S's. Just my guess, but don't go hoping too bad for the 64 GB 5S. That one may be a fossil from now on.
Ugh!
I was inches away from ordering the gold 64 GB 6 Plus, even though I wanted the space grey, purely out of frustration of waiting) before I saw that that too is now sold out.
I have my little chrome monitor extension checking on the website, but who knows when any 64's will be back in stock...
I hate it when that happens.
But nowadays you can just log onto your online banking and buy prepaid airtime there.
Plus, on contract you pay more per minute than on prepaid, which is the opposite of how it should be.