Not a fan of this policy, and it has soured me to using Amazon.co.jp, but I guess people have been abusing the preorder system, by ordering and then cancelling often, like we see with BBTS I guess; the 'cancellation window' was fairer in my opinion though.

 

If @Smokescreen38 has anything else using that card, this method will create extra hassle. I'd save this approach as a last resort or a method of completely ensuring that the order won't go through.

When the preordered item is available and Amazon JP begins the shipping process, they'll first attempt to charge the card you have on file in your AJP account. If it can't be charged for any reason (card expired, insufficient balance, etc) then AJP sends you an email stating that they couldn't charge you for the item and that you have six days to update your payment method. If a valid payment method isn't added to your account within that six days they automatically cancel the order. That's it.

You have a few options here:

1. If your card happens to expire before December, just leave it as is and don't update the expiration date in your AJP payment methods.
   
2. Try seeing if you can completely remove your card from your Amazon JP account. I don't know if this will work if you have an item on preorder. They may say that you need a payment method in your account and may not let you remove the card. If, however, you can remove the card then your problem is solved.
3. If you have another card that won't have a sufficient amount of credit to fulfill the order, add that card to your payment options and remove the other card. Make sure that card still won't cover the preorder at the end of December. You may be able to use a prepaid Visa card with a low balance for this if you don't have another credit card.
4. If none of the above work or are possible for you, call the bank that issued your card and have them block transactions from Amazon Japan. Once the order is cancelled call them again and remove the block.

They still have the cancellation window, it's just much shorter than it used to be.