Got my mugen cover a few days ago.
I had just made up an order number using nine 0's, which was accepted by their form. A few days went by without confirmation from Mugen, but they told me they would ship it after I enquired by email.
Others might have even better luck with making up an order number in the "real" range of 50002XXXX (replace the x's with some small random number, less than 3000)