Somehow I accidentally changed stores and now can't change back

I Was looking for an ebook, and got the message that it wasn't available on the Australian store, but was in the US one. I thought I could change back easily if I let it change stores so I could see the book. Now, I can't switch back to the Australian store. And I can't make purchases, because I'm registered in the Australian store.

OH, and an active iTunes Match subscription isn't helping--apparently it's the reason I can't change stores.