Fortunately, Blizzard is on the case:
We're experiencing difficulties with pre-orders for Reaper of Souls. These issues will be resolved ASAP. Thanks very much for your patience!- BlizzardCS (@BlizzardCS) March 21, 2014
Hopefully we'll see a resolution sooner rather than later -- with Reaper of Souls due out on the 25th, there's not much time left to buy for release-day gameplay. In the meantime, we're holding off on clicking "buy" until @BlizzardCS gives us the all clear.