My initial attempt to buy the plugin had failed - after filling out the form it just dumped me back to the main page. I figured I'd probably just saved myself $14 and forgot about it.
Then MF sent me an email saying I had an item in my cart, with a link to finish the transaction. OK, I thought. Fate does want me to buy it. So I clicked on the provided link and it took me back to the MF main page again. Because I had registered as "Guest" and therefore have no account to log in to, there is no way to actually access my shopping cart. The email warned me that I could not reply to it, so even though they know who I am there is no way to actually complete the transaction. I guess my credit card number will persist there forever in digital limbo, at least until some database administrator clears out the uncommitted transactions during annual maintenance.
Don't you just love this fully-automated human-free retail world we all now enjoy? Progress...nobody said it would be easy.