Hi Matthew,
I’m sorry to hear you’ve hit up against some problems.
I can’t actually replicate the problem so we’ll need to gather a little more information. If you are comfortable doing so, could you enable WP_DEBUG mode by adding the following snippet to your wp-config.php file:
define( 'WP_DEBUG', true );
If you’re unsure about the correct place to add this, do check out this codex article. With that done, can you return to the order screen: are any errors displayed and can you quote them/screenshot them if so?
Feel free to undo your change to wp-config.php once you’ve finished testing.
Thanks so much!